Method and system employing multiple reflectivity band reflector for laser wavelength monitoring

ABSTRACT

A monitored laser system includes a laser with a first mirror and an exit mirror. The laser also has a laser cavity defined at least in part by the first mirror and the exit mirror. Within the laser cavity is an active region that contains material that is capable of stimulated emission at one or more wavelengths such that laser light is emitted from the laser. A power source is coupled to the active region. A multiple reflectivity band reflector (MRBR) is coupled to at least a portion of the emitted laser light. The MRBR has at least first and second wavelength bands with reflectivity above a particular reflectivity separated by at least a third wavelength band having reflectivity below the particular reflectivity. A first photodiode is coupled to at least a portion of the filtered laser light and produces an output based on the amount and wavelength of light received. A means for adjusting the emitted wavelength of the laser toward a particular wavelength in one of the at least first, second, and third wavelength bands based at least in part on the output of the first photodiode.

CROSS-REFERENCES TO RELATED APPLICATIONS

This nonprovisional U.S. national application, filed under 35 U.S.C.§111(a), claims, under 37 C.F.R. §1.78(a)(3), the benefit of the filingdate of provisional U.S. national application Nos. 60/272,707,60/272,710, 60/272,627 and 60/272,622, each filed under 35 U.S.C.§111(b) and accorded a filing date Mar. 1, 2001, the entireties of eachof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to devices that emit electromagneticradiation and, in particular, to wavelength monitoring and locking for asemiconductor laser.

2. Description of the Related Art

The following descriptions and examples are not admitted to be prior artby virtue of their inclusion within this section.

Lasers have a wide range of industrial and scientific uses. There areseveral types of lasers, including gas lasers, solid-state lasers,liquid (dye) lasers, and free electron lasers. Semiconductor lasers arealso in use. In semiconductor lasers, electromagnetic waves areamplified in a semiconductor superlattice structure. Semiconductorlasers may be diode lasers (bipolar) or non-diode lasers such as quantumcascade (QC) lasers (unipolar). Semiconductor lasers are used for avariety of applications and can be built with different structures andsemiconductor materials, such as gallium arsenide (GaAs).

The use of semiconductor lasers for forming a source of optical energyis attractive for a number of reasons. Semiconductor lasers have arelatively small volume and consume a small amount of power as comparedto conventional laser devices. Further, semiconductor lasers can befabricated as monolithic devices, which do not require a combination ofa resonant cavity with external mirrors and other structures to generatea coherent output laser beam.

A semiconductor laser typically comprises an active (optical gain)region sandwiched between two mirrors (reflectors or reflective planes).There is typically a small difference in reflectivity between the twomirrors, one of which (typically, the reflective plane having lowerreflectivity) serves as the “exit” mirror. The area between thereflective planes is often referred to as the resonator, or theFabry-Perot resonance cavity in some cases. The active region is locatedwithin the resonant cavity. When the active region is pumped with anappropriate pumping energy, it produces photons, some of which resonateand build up to form coherent light in the resonant cavity formed by thetwo mirrors. A portion of the coherent light built up in the resonatingcavity formed by the active region and top and bottom mirrors passesthrough the exit mirror as the output laser beam.

Various forms of pumping energy may be utilized to cause the activeregion to begin to emit photons and to achieve gain. For example,semiconductor lasers of various types may be electrically pumped (EP)(by a DC or alternating current), or pumped in other ways, such as byoptical pumping (OP) or electron beam pumping. In an EP VCSEL, forexample, an electrical potential difference is typically applied acrossthe active region (via top and bottom electrical contacts provided aboveand below the active region). As a result of the potential applied, apumping current flows through the active region, i.e. charge carriers(electrons and holes) are injected from opposite directions into theactive region where recombination of electron and holes occurs. Thereare two kinds of recombination events, i.e. radiative and non-radiative,concurrently happening in the active region. When radiativerecombination occurs, a photon is emitted with the same energy as thedifference in energy between the hole and electron energy states. Someof those photons travel in a direction perpendicular to the reflectorsof the laser. As a result of the ensuing reflections, the photons cantravel through the active region multiple times.

Stimulated emission occurs when radiative recombination of anelectron-hole pair is stimulated by interaction with a photon. Inparticular, stimulated emission occurs when a photon with an energyequal to the difference between an electron's energy and a lower energyinteracts with the electron. In this case, the photon stimulates theelectron to fall into the lower energy state, thereby emitting a secondphoton. The second photon will have the same energy and frequency as theoriginal photon, and will also be in phase with the original photon.Thus, when the photons produced by spontaneous electron transitioninteract with other high energy state electrons, stimulated emission canoccur so that two photons with identical characteristics are present.(Viewed as waves, the atom emits a wave having twice the amplitude asthat of the original photon interacting with the atom.) If a sufficientamount of radiative recombinations are stimulated by photons, the numberof photons traveling between the reflectors tends to increase, givingrise to amplification of light and lasing. The result is that coherentlight builds up in the resonant cavity formed by the two mirrors, aportion of which passes through the exit mirror as the output laserbeam.

Semiconductor lasers may be edge-emitting lasers or surface-emittinglasers (SELs). Edge-emitting semiconductor lasers output their radiationparallel to the wafer surface, while in SELs, the radiation output isperpendicular to the wafer surface. One type of SEL is thevertical-cavity surface-emitting laser (VCSEL). The “vertical” directionin a VCSEL is the direction perpendicular to the plane of the substrateon which the constituent layers are deposited or epitaxially grown, with“up” being typically defined as the direction of epitaxial growth. Insome designs, the output laser beam is emitted out of the top side, inwhich case the top mirror is the exit mirror. In other designs, thelaser beam is emitted from the bottom side, in which case the bottommirror is the exit mirror.

VCSELs have many attractive features compared to edge-emitting lasers,such as low threshold current, single longitudinal mode, a circularoutput beam profile, a smaller divergence angle, and scalability tomonolithic laser arrays. The shorter cavity resonator of the VCSELprovides for better longitudinal mode selectivity, and hence narrowerlinewidths. Additionally, because the output is perpendicular to thewafer surface, it is possible to test fabricated VCSELs on the waferbefore extensive packaging is done, in contrast to edge-emitting lasers,which must be cut from the wafer to test the laser. Also, because thecavity resonator of the VCSEL is perpendicular to the layers, there isno need for the cleaving operation common to edge-emitting lasers.

The VCSEL structure usually consists of an active (optical gain) regionsandwiched between two mirrors, such as distributed Bragg reflector(DBR) mirrors. Both EP and OP VCSEL designs are possible. The twomirrors may be referred to as a top DBR and a bottom DBR. Because theoptical gain is low in a vertical cavity design, the reflectors requirea high reflectivity in order to achieve a sufficient level of feedbackfor the device to laser.

DBRs are typically formed of multiple pairs of layers referred to asmirror pairs. DBRs are sometimes referred to as mirror stacks. The pairsof layers are formed of a material system generally consisting of twomaterials having different indices of refraction and being easilylattice matched to the other portions of the VCSEL, to permit epitaxialfabrication techniques. The layers of the DBR are quarter-waveoptical-thickness (QWOT) layers of alternating high and low refractiveindices, where each mirror pair contains one high and one low refractiveindex QWOT layer. The number of mirror pairs per stack may range from20-40 pairs to achieve a high percentage of reflectivity, depending onthe difference between the refractive indices of the layers. A largernumber of mirror pairs increases the percentage of reflected light(reflectivity).

The DBR mirrors of a typical VCSEL can be constructed from dielectric(insulating) or semiconductor layers (or a combination of both,including metal mirror sections). The difference between the refractiveindices of the layers of the mirror pairs can be higher in dielectricDBRs, generally imparting higher reflectivity to dielectric DBRs than tosemiconductor DBRs for the same number of mirror pairs and overallthickness. Conversely, in a dielectric DBR, a smaller number of mirrorpairs can achieve the same reflectivity as a larger number in asemiconductor DBR. However, it is sometimes necessary or desirable touse semiconductor DBRs, despite their lower reflectivity/greaterthickness, to conduct current, for example (e.g., in an EP VCSEL).Semiconductor DBRs also have higher thermal (heat) conductivity than dodielectric DBRs, making them more desirable for heat-removal purposes,other things being equal. Semiconductor DBRs may also be preferred formanufacturing reasons (e.g., a thicker DBR may be needed for support) orfabrication reasons (e.g., an epitaxial, i.e. semiconductor, DBR may beneeded if other epitaxial layers need to be grown on top of the DBR).

When properly designed, these mirror pairs will cause a desiredreflectivity at the laser wavelength. Typically in a VCSEL, the mirrorsare designed so that the bottom DBR mirror (i.e. the one interposedbetween the substrate material and the active region) has nearly 100%reflectivity, while the top (exit) DBR mirror has a reflectivity thatmay be 98%-99.5% (depending on the details of the laser design). Thepartially reflective top (exit) mirror passes a portion of the coherentlight built up in the resonating cavity formed by the active region andtop and bottom mirrors. Of course, as noted above, in other designs, thebottom mirror may serve as the exit mirror and the top mirror has thehigher reflectivity. VCSELs, DBRs, and related matters are discussed infurther detail in Vertical-Cavity Surface-Emitting Lasers: Design,Fabrication, Characterization, and Applications, eds. Carl W. Wilmsen,Henryk Temkin & Larry A. Coldren (Cambridge: Cambridge University Press,1999).

In standard VCSELs, the active region and top and bottom mirrors aremonolithically fabricated on a substrate. A variant on the standardVCSEL, an external cavity VCSEL, or vertical-external-cavitysurface-emitting laser (VECSEL), is also in use. In this case, theactive region and bottom mirror are monolithically fabricated on asubstrate, while the top mirror is mounted externally, some distance(typically very small) above the active region. VECSELs are described inJ. Sandusky & S. Brueck, “A CW External-Cavity Surface-emitting Laser,”IEEE Photon. Techn. Lett. 8, 313-315 (1996). The term VCSEL may be usedto refer to both standard (monolithic) VCSELs and external-cavity VCSELS(VECSELs).

VCSEL characteristics are capable of extensive modeling andmanipulation. Sarzala et al., “Carrier Diffusion Inside Active Regionsof Gain-Guided Vertical-Cavity Surface-Emitting Lasers,” IEEEProc.—Optoelectonics, vol.144, no.6, p.421-24, December 1997, Langley etal., “Effect of Optical Feedback on the Noise Properties of VerticalCavity Surface Emitting Lasers,” IEEE Proc.—Optoelectonics, vol. 144,no. 1, p. 34-38, February 1997, Ha et al., “Polarisation Anisotropy inAsymmetric Oxide Aperture VCSELs,” Electronics Letters, vol. 34, no. 14,July 1998.

Semiconductor lasers such as VCSELs and edge-emitting lasers are used ina variety of applications. In some applications, e.g.,telecommunications and spectroscopy among others, the output laser lightis modulated to achieve the objective of the system. Modulation consistsof modifying a characteristic of the laser output, e.g., the amplitude,frequency, or phase. In the case of telecommunications, the modulationsare patterned to correspond to information. The laser may be externallymodulated, or directly modulated. When the radiation of the output laserbeam is detected after it has traveled to another point, the modulationsindicate the information that was encoded at the transmitter/modulatorend. A typical telecommunications system uses optical fiber to guide theradiation from the modulation (or emission) point to the detectionpoint. Long wavelength (1.3 μm to 1.55 μm) VCSELs, for example, are ofgreat interest in the optical telecommunications industry because of theminimum fiber dispersion at 1310 nm and the minimum fiber loss at 1.55μm (1550 nm).

It is important to be able to monitor, and sometimes control, thewavelength of the emitted laser radiation in some applications. Intelecommunications applications, for example, the emitted laserradiation of a given semiconductor laser has a precise wavelength, asspecified, for example, by the ITU grid. For example, the ITU gridspecifies lasing wavelength of 1.55 μm (and other closely spacedwavelengths). These ITU grid wavelengths are used in telecommunicationsapplications such as coarse and dense wavelength-division multiplexing(CWDM and DWDM). In WDM, typically used in optical fiber communications,two or more optical (e.g. laser) signals having different wavelengthsare simultaneously transmitted in the same direction over one fiber, andthen are separated by wavelength at the distant end.

The use of wavelength-division multiplexed communications systems hasled to additional equipment. For example, devices for demultiplexing thewavelengths include the disclosure of U.S. Pat. No. 5,894,535 (1999),Lemoff et al., “Optical Waveguide Device for Wavelength Demultiplexingand Waveguide Crossing.” That patent discloses a device including azigzag patterned dielectric channel waveguide structure that guides aWDM signal through a zigzag path. At particular vertices of the pathoptical filters selectively transmit and reflect wavelengths of light.The light output of the device separates wavelength of light by outputposition. As another example, U.S. Pat. No. 5,673,129 (1997), Mizrahi,“WDM Optical Communication Systems with Wavelength Stabilized OpticalSelectors” discloses a system that receives a portion of a WDM signalwith a Bragg grating having one high reflectivity wavelength band. Basedon the signal that is reflected from the grating, a wavelength parameterof the Bragg grating is modified, resulting in a change in the highreflectivity wavelength band. A system basing feedback on the signaltransmitted by the grating is also disclosed. U.S. Pat. No. 6,111,681(2000), Mizrahi et al., “WDM Optical Communication Systems withWavelength Stabilized Optical Selectors” contains the same disclosure asU.S. Pat. No. 5,673,129.

Systems that adjust the output wavelength of a laser have also beenproposed. For example, U.S. Pat. No. 5,943,152, Mizrahi et al., “LaserWavelength Control Device” discusses a system that couples an in-fiberBragg grating to the output of a laser. Based on either thetransmissivity or reflectance of the grating, a microprocessorcontinually adjusts the wavelength of the laser output. As anotherexample, U.S. Pat. No. 5,875,273, Mizrahi et al., “Laser WavelengthControl Under Direct Modulation” discusses a system using a filter withparticular transmission characteristics as a function of wavelength. Inparticular, the filter includes a transmissivity minimum withtransmissivity maximums for both a greater and lesser wavelength, whichcan also be described as a high reflectivity wavelength band. The filteris coupled to a laser and a control circuit adjusts the laser'swavelength characteristics based on measurement of both reflected andtransmitted light from the filter. As another example, U.S. Pat. No.6,067,181, Mizrahi, “Laser Locking and Self Filtering Device” discussesa laser system with an optical transfer element and a Bragg grating. Theentire output of the laser is coupled to the Bragg grating via thetransfer element. The light reflected from the Bragg grating isoutputted while the light transmitted through the Bragg grating isdetected to generate a signal that is used to control the laser. U.S.Pat. No. 6,125,128 (2000), Mizrahi, “Laser Output Locking and SelfFiltering Device” contains substantially the same disclosure as U.S.Pat. No. 6,067,181.

It can be difficult to ensure that a given laser is lasing at thedesired wavelength, and to control or tune the laser to emit atdifferent wavelengths. For example, VCSELs can have a wavelengthsignificantly dependent on drive current (or some other tuningparameter), and can be thus said to be “tunable”. In general, a tunablelaser such as a tunable VCSEL is a laser having an output wavelengthcorresponding to a selectable tuning parameter. Some approaches used inattempts to tune various types of lasers are described in B. Pezeshki,“New Approaches to Laser Tuning,” Optics & Photonics News, 34-38 (May2001). These include, in addition to varying the pumping or drivecurrent, temperature variation, combination of multiple lasers havingdifferent wavelengths on a single chip, and movement of micromechanicalcomponents.

However, while tunability is desired in some applications, it can giverise to undesired variation in lasing wavelength. Additionally, evenlasers that initially have a fixed or stable wavelength can havewavelength drift over time, as the device ages. It can be important tobe able to determine that desired wavelength, or wavelength “lock,” hasbeen lost. Information about deviation of the current wavelength fromsome benchmark or target wavelength can be useful for diagnostic orlocking purposes, for example.

There is, therefore, a need for methods and devices to permitmonitoring, stabilizing, selecting, and controlling the lasingwavelength of semiconductor lasers.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent uponstudy of the following description, taken in conjunction with theattached FIGS. 1-16.

FIG. 1 is a chart of reflectance as a function of wavelength for areflector.

FIG. 2A is a chart of reflectance as a function of wavelength for areflector constructed in accordance with FIG. 2C.

FIG. 2B is a chart of reflectance as a function of wavelength for areflector constructed in accordance with FIG. 2C.

FIG. 2C is a diagram of a reflector.

FIG. 3A is a chart of reflectance as a function of wavelength for areflector constructed in accordance with FIG. 3C.

FIG. 3B is a chart of reflectance as a function of wavelength for areflector constructed in accordance with FIG. 3C.

FIG. 3C is a diagram of a reflector.

FIG. 4A is a chart of reflectance as a function of wavelength for areflector constructed in accordance with FIG. 4C.

FIG. 4B is a chart of reflectance as a function of wavelength for areflector constructed in accordance with FIG. 4C.

FIG. 4C is a diagram of a reflector.

FIG. 5A is a chart of reflectance as a function of wavelength for areflector constructed in accordance with FIG. 5B.

FIG. 5B is a diagram of a reflector.

FIG. 6A is a chart of reflectance as a function of wavelength for areflector constructed in accordance with FIG. 6D.

FIG. 6B is a chart of reflectance as a function of wavelength for afirst implementation of a reflector constructed in accordance with FIG.6D.

FIG. 6C is a chart of reflectance as a function of wavelength for afirst implementation of a reflector constructed in accordance with FIG.6D.

FIG. 6D is a diagram of a reflector.

FIG. 7A is a chart of reflectance as a function of wavelength for asecond implementation of a reflector constructed in accordance with FIG.6D.

FIG. 7B is a chart of reflectance as a function of wavelength for asecond implementation of a reflector constructed in accordance with FIG.6D.

FIG. 8 is a diagram of a system for monitoring a laser.

FIG. 9 is a diagram of a system for monitoring a laser.

FIG. 10A is a diagram of a system for monitoring an external cavitylaser.

FIG. 10B is a diagram of an external cavity laser.

FIG. 11 is a diagram of a vertical cavity surface emitting laser.

FIG. 12 is a diagram of a system for monitoring a vertical cavitysurface emitting laser.

FIG. 13 is a diagram of a system for monitoring a laser.

FIG. 14 is a flowchart depicting a method of configuring a tunablelaser.

FIG. 15 is a flowchart depicting a method of changing wavelength band ina tunable laser.

FIG. 16 is a diagram of zigzag waveguide device-based wavelength lockingapparatus.

While the present invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the present invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, the details of preferred embodiments ofthe invention are schematically illustrated.

Multiple Reflectivity Band Reflector

A reflector is fabricated having a structure that gives rise to multiplehigh, and narrow, reflectivity bands. Such a reflector may be referredto as a multiple reflectivity-band reflector (MRBR). In an embodiment,the reflectivity bands of the MRBR are spaced along wavelengths of theITU grid, e.g. around 1550 nm. In another embodiment, additionalreflectivity bands of the MRBR are spaced around 1310 nm. In anembodiment, the MRBR is a distributed dielectric multilayer stackreflector.

Each reflectivity band has a peak reflectivity and an associated peakwavelength, and is separated from adjacent reflectivity bands bywavelength bands which may be referred to as “troughs”. The peaks of thereflectivity bands, as well as the minima (bottom tips) of the troughsbetween bands (i.e. transmissivity peaks), of an MRBR, have a certainreflectivity profile (envelope). In the present application, the term“reflectivity profile” refers to the shape of the function intersectingthe peaks of bands (or the trough minima of troughs between bands), fora given mirror's reflectance versus wavelength function (reflectancecharacteristic). The reflectivity profile of the peaks of reflectancebands may be referred to as the peak profile, while the trough profilerefers to the reflectivity profile of the trough minima. Thereflectivity profile may also be referred to as the “envelope function”or profile for the peaks or trough minima of a given reflectancecharacteristic. In some embodiments, the reflectivity profile (e.g., ofthe reflectivity band peaks) is substantially constant over a givenwavelength range (as described in further detail below with reference toFIGS. 1-4). In other embodiments, the peak profile varies as a functionof wavelength (as described in further detail below with reference toFIG. 5).

Depending on the embodiment, either all, some, or none of thereflectivity bands have a reflectivity sufficient to give rise tolasing. Reflectivity bands that do have sufficient reflectivity topermit lasing may be referred to as lasing threshold reflectivity bands.

Each reflectivity band has a peak reflectivity and a narrow wavelengthband encompassing the peak, in which band all the reflectivities areabove some reflectivity threshold. Between adjacent peaks or bands aretroughs in which the reflectivity is below this threshold reflectivity.

In an embodiment, the reflectivity of a plurality of reflectivity bandsof the MRBR is high enough to give rise to lasing in a VCSEL ortwo-section VECSEL configuration employing the MRBR. In this case, thereflectivity bands are lasing threshold reflectivity bands. Each lasingthreshold reflectivity band has a reflectivity, in a very narrowwavelength band, above a certain minimum or lasing thresholdreflectivity. This threshold reflectivity is high enough so that thereis net gain at that wavelength (i.e., the gain exceeds the mirror loss).The lasing threshold reflectivity band thus denotes the narrow range ofwavelengths centered about the peak reflectivity wavelength for thereflectivity band and for which the reflectivity is above some lasingthreshold reflectivity.

Lasing occurs when, during a round trip of photons through the cavity,the number of photons added due to stimulated emission is at least equalto the number lost internally and at the edges. A lower reflectivity fora given reflectivity band leads to more photons being lost at the “edge”of the cavity formed by the MRBR. Thus, lasing is possible where gain(determined by the gain spectrum of the active region) is greater thanloss (determined in part by the reflectivity of the DBR mirrors), andwhere the phase difference of a round trip of light within the opticalcavity is zero. A wavelength where the phase difference of a round tripof light within the optical cavity is zero may be referred to herein asa cavity mode or a zero-phase difference wavelength. The precise cavitymodes for a given laser are determined by the physical distance betweenthe mirrors and by the reflectivity and phase shifting characteristicsof the DBR mirrors, and by the indices of refraction of various layersof material within the laser structure. The cavity modes can be shiftedby adjusting some indices of refraction within the laser cavity, bychanging the physical cavity length, or other techniques.

This minimum reflectivity which is sufficient for lasing in a givenlaser structure may be referred to as the lasing threshold reflectivityand may be, for example, 99.5%. Each reflectivity peak that is above thelasing threshold reflectivity is within an associated lasing thresholdreflectivity band. Between any two adjacent lasing thresholdreflectivity bands the reflectivity is necessarily below the lasingthreshold reflectivity. As noted above, the troughs are portions of themirror's reflectivity profile between any two adjacent reflectivitybands. As will be appreciated, a trough between two adjacent lasingthreshold reflectivity bands will have a reflectivity lower than thelasing threshold reflectivity over the entire trough, and for virtuallyall of the trough (except for the portion immediately adjacent thebottom portions of the adjacent reflectivity bands) lower than a secondthreshold reflectivity (e.g., 99.3%) which is lower than the lasingthreshold reflectivity.

The reflectivity bands of the MRBR are preferably sufficiently narrow.The reflectivity band may be characterized in terms of narrowness withrespect to the wavelength band covered by a given reflectivity, e.g. thefirst threshold reflectivity, or other specified high reflectivity(e.g., 99%). The wavelength range covered by a reflectivity band abovesome first threshold reflectivity may be denoted Δλ_(TR) (where “TR”stands for first “threshold reflectivity”), and the range covered withgreater than 99% reflectivity may be denoted Δλ₉₉. Where the firstthreshold reflectivity is a lasing threshold reflectivity (i.e., wherethe reflectivity band is a lasing threshold reflectivity band), thedenotation Δλ_(LT) may be employed instead of Δλ_(TR), where “LT” standsfor “lasing threshold.” The reflectivity band may also described ashaving a certain width at a reflectivity a certain percentage (e.g., 3%)below its peak reflectivity. For example, in an embodiment, eachreflectivity band has a width of less than 1 nm at a reflectivity of 3%less than its peak reflectivity. Alternatively, in an embodiment, agiven reflectivity band may be described as having Δλ₉₉ less than orequal to some width (e.g., 1 nm, or 0.1 nm, etc.).

The particular lasing threshold reflectivity depends uponcharacteristics of the laser cavity such as the reflectivity of theother edge (of the cavity—whether a cleaved edge or DBR or other type ofreflector, e.g.), the degree to which the active region absorbsparticular wavelengths of light, and the degree to which atoms in theactive region are stimulated to emit additions photons at particularwavelengths of light (i.e., the gain). For example, if the bottom mirrorof a VECSEL has very high reflectivity, then lasing thresholdreflectivity for the other mirror (e.g., the MRBR) is lower. Conversely,a higher lasing threshold reflectivity is needed for the top or externalmirror (e.g., the MRBR) given a lower reflectivity bottom cavity mirror.Lasing can occur where there is a cavity mode, at a wavelength lyingwithin a given reflectivity band (i.e., at a cavity mode wavelength atwhich the reflectivity of the reflectivity band exceeds the lasingthreshold reflectivity).

FIG. 1 plots the reflectance versus wavelength characteristics for anexemplary uniform reflectivity profile MRBR in accordance with thepresent invention. The reflectance is determined with unpolarized lightimpacting the MRBR perpendicularly through air. The reference wavelengthis 2000 nm. The “reference wavelength” term specifies the wavelengthused to determine the QWOT for the layers of the MRBR; i.e., they have athickness that is equal to a quarter of the reference wavelength in thatmaterial. The MRBR with the reflectance characteristics shown in FIG. 1has a total thickness of approximately 212.4 μm, and may be fabricatedon a suitable substrate, such as glass or InP. As will be appreciated,various multilayer stack structures and materials may be utilized toresult in reflectivity bands of various wavelengths, spacing, and width(narrowness), and having different reflectivities for the peaks andtrough minima, and various constant or varying peak and trough minimaprofiles.

Referring now to FIGS. 2A-B, there are shown plots of the reflectanceversus wavelength characteristics for another exemplary MRBR inaccordance with the present invention, the structure of which is shownin FIG. 2C. This MRBR also has a substantially uniform reflectivityprofile for at least a plurality of peaks. FIG. 2B graphs a much smallerportion of both axes than is shown in FIG. 2A. For both FIG. 2A and FIG.2B, the reflectance is determined with unpolarized light impacting theMRBR perpendicularly through air. The MRBR reflectance characteristicsshown in FIGS. 2A-B may be achieved by employing an MRBR having a layerstructure either substantially similar to or based on/derived from thestructure shown in FIG. 2C and defined by the following Formula (1):

EABCD(ABC)⁶⁰ABCF(ABC)⁶⁰AB  (1)

where an exponent indicates that the preceding term is repeated thatmany times in succession, the letters “qw” denote quarter-wave opticalthickness (QWOT) (for a given reference wavelength), and the layersymbols in the formula indicate the following materials and thicknesses:

Symbol Material Thickness A Al₂O₃ 0.7500 qw B TiO₂ 0.7500 qw C SiO₂0.7500 qw D Si 0.7500 qw E Al 1.2500 qw F Si 0.7500 qw

The reference wavelength is 1800 nm. For the simulations used togenerate the plots for the MRBRs of the present invention, the qwfigures were based on the following assumed indices of refraction andextinction coefficients:

Material Refractive Index Extinction Coefficient Al₂O₃ 1.62 0 TiO₂ 2.3 0SiO₂ 1.434 0 Si 3.4 0 Al 2.3 16.5

For example, the A layer has a quarter wavelength equal to the referencewavelength divided by both the refractive index of the material and by4: (1800 nm/1.62)/4=277.778 nm. Division by the refractive indexaccounts for the fact that the wavelength of light depends on thepropagating medium. The A layer is therefore 0.75*qw =0.75*277.778nm=208.33 (208) nm thick. The same calculation produces the thicknessesof the other layers:

Symbol Material Thickness A Al₂O₃ 208.33 nm B TiO₂ 146.74 nm C SiO₂235.36 nm D Si  99.26 nm E Al 244.57 nm F Si  99.26 nm

As noted above, to obtain the desired MRBR reflectance characteristics(e.g., those shown in FIGS. 2A-B), an MRBR may be employed having alayer structure either substantially similar to or based on/derived fromthe structure defined Formula (1). For example, in an embodiment, toarrive at the actual MRBR layer structure to be employed, the layerstructure of Formula (1) is used as a starting point, to result in amodified layer structure similar to and/or based on the initial layerstructure specified in Formula (1). For example, the reflector structuremay be optimized by varying the individual layer thicknesses so as toachieve the precise reflectance characteristics desired. Thus, althoughFormula (1) indicates many layers having identical thickness, afteroptimization the reflector utilized may have layers of differentthicknesses. Whether the MRBR employed has a layer structuresubstantially similar to, or based on/derived from a structure definedby a given formula, the MRBR may be said to have a layer structure“based on”, “derived from,” or “corresponding to” the layer structuredefined by the formula.

In alternative embodiments or with other fabrication processes orsputtering devices, the refractive indices and extinction coefficientsof the layer materials employed may vary somewhat from those assumedabove. In this case, the qw figures might need to be adjusted, to takeinto account actual indices of refraction, to achieve a givenreflectivity characteristic. The qw parameter for each of the simulatedplots of FIGS. 1-7 is the QWOT for a given reference wavelength, denoted“reference” on the legend at the top of each of FIGS. 1-7. Thus, in someembodiments, due to optimization or other empirical adjustments, theformula itself may be adjusted or the implementation may be adjusted.For example, for Formula (1), layer A appears several times in theformula. Although nominally indicated as being an Al₂O₃ having athickness of 0.7500 qw (or 208.33 nm), this thickness of layer A may bevaried, either for all the A layers in the structure or only some ofthem.

Formula (1) specifies a 371-layer stack, where the leftmost symboldenotes the layer(s) closest to the substrate. The substrate is notshown in FIG. 2C. The reflector may be fabricated by any suitabletechnique, e.g., a dielectric coating technique such as such as ion beamassisted sputtering or magnetron sputtering on a suitable substrate,such as InP or glass. In some cases, the reflector need not use asupporting substrate, i.e. it has an “air” substrate.

The particular stack layer formula employed may be empiricallydetermined, e.g. with the aid of a suitably programmed computer (e.g.,running the TFCalc program available from Software Spectra, Inc., havinga web site with a www domain name of sspectra.com) which attempts tofind suitable layer structures, within certain constraints, to yield thedesired reflectivity band characteristics. The qw parameter may beselected for a reference wavelength greater than or less than the actualdesired wavelength range for the reflectivity bands. For example,reference wavelengths of 1800, 2000, 2200, and 795 nm were employed, insome cases, to achieve reflectivity bands in the 1550 nm range.

As noted above, alternate layer structures may be employed to achievedesired reflectivity band characteristics. Referring now to FIGS. 3A-B,there are shown plots of the reflectance versus wavelengthcharacteristics for another exemplary MRBR in accordance with thepresent invention and diagrammed in FIG. 3C. FIG. 3B graphs a muchsmaller portion of both axes than is shown in FIG. 3A. For both FIG. 3Aand FIG. 3B, the reflectance is determined with unpolarized lightimpacting the MRBR perpendicularly through air. The MRBR reflectancecharacteristics shown in FIGS. 3A-B may be achieved by employing an MRBRhaving a layer structure either substantially similar to or basedon/derived from the structure shown in FIG. 3C and defined by thefollowing formula:

EABCD(ABC)⁶⁰ABCF(ABC)⁶⁰AG  (2)

where the layer symbols in the formula indicate the following materialsand thicknesses:

Symbol Material Thickness A Al₂O₃ 0.7500 qw B TiO₂ 0.7500 qw C SiO₂0.7500 qw D Si 0.7500 qw E Al 1.0000 qw F Si 0.7500 qw G Si 0.7500 qw

The reference wavelength is 1800 nm. By performing the thicknesscalculation, the following thicknesses are obtained:

Symbol Material Thickness A Al₂O₃ 208.33 nm B TiO₂ 146.74 nm C SiO₂235.36 nm D Si  99.26 nm E Al 244.57 nm F Si  99.26 nm G Si  99.26 nm

This MRBR has a total thickness of approximately 72.75 μm. Inembodiments with different optical characteristics, resulting forexample from different deposition techniques, different measurementswill be appropriate. In one embodiment the MRBR is fabricated on an InPsubstrate.

As can be seen, the MRBRs of FIGS. 2 and 3 each have reflectivity bandsclosely spaced, around the 1550 ITU grid wavelengths, withreflectivities exceeding 99.98%. The troughs have lower reflectivities.In an embodiment, the MRBR comprises a plurality of reflectivity bands,each covering one of a contiguous set of 1550 ITU grid wavelengths,preferably where each reflectivity band has a peak reflectivity at orvery close to the particular ITU grid wavelength covered by thatreflectivity band. In other embodiments, the MRBR reflectivity bands maycover non-contiguous ITU grid wavelengths over a certain wavelengthrange (i.e., there are some ITU grid wavelengths not covered by areflectivity band, in which lasing/monitoring is possible only for thoseITU grid wavelengths for which there is a reflectivity band). In stillother embodiments, over a given wavelength range, there may be somereflectivity bands that lie between ITU grid-covering reflectivitybands, i.e. there are more reflectivity bands than necessary to coverthe ITU grid wavelengths in the range, in which case the “extra”reflectivity bands may be ignored or unused, depending on theapplication.

Referring now to FIGS. 4A-B, there are shown plots of the reflectanceversus wavelength characteristics for another exemplary MRBR inaccordance with the present invention and shown in FIG. 4C. FIG. 4Bgraphs a much smaller portion of the reflectivity axis than is shown inFIG. 4A. For both FIG. 4A and FIG. 4B, the reflectance is determinedwith unpolarized light impacting the MRBR perpendicularly through air.The MRBR reflectance characteristics achieved by the MRBR shown in FIGS.4A-B may be achieved by employing an MRBR having a layer structureeither substantially similar to or based on/derived from the structureshown in FIG. 4C and defined by the following formula:

ABC(ABCD)²(ABDC)⁶⁸(ABCD)  (3)

where the layer symbols in the formula indicate the following materialsand thicknesses:

Symbol Material Thickness A Al₂O₃ 0.7500 qw B TiO₂ 0.7500 qw C SiO₂0.7500 qw D Si 1.0000 qw

The reference wavelength is 2200 nm. By performing the thicknesscalculation, the following thicknesses are obtained:

Symbol Material Thickness A Al₂O₃ 254.63 nm B TiO₂ 179.35 nm C SiO₂287.66 nm D Si 161.75 nm

This MRBR may be fabricated on an InP substrate, in an embodiment.

As will be appreciated from the Figures, the MRBRs of FIGS. 1-4 havemultiple, relatively uniformly spaced reflectivity bands and peaks,where each reflectivity band has a reflectivity above the lasingthreshold reflectivity, separated by reflectivity troughs, in which thereflectivity for all wavelengths in the wavelength band of the troughare lower than the lasing threshold reflectivity. As noted above, theparticular lasing threshold reflectivity depends upon characteristics ofthe laser cavity such as the reflectivity of the other edge, the degreeto which the active region absorbs particular wavelengths of light, andthe degree to which atoms in the active region are stimulated to emitadditions photons at particular wavelengths of light (gain). Thesecharacteristics can be modified under some circumstances. For example,the degree to which the active region is electrically pumped affects theoptical gain (gain spectrum). In an embodiment, the lasing thresholdreflectivity for a plurality of reflectivity bands may be the lasingthreshold reflectivity with respect to the gain at or near its maximum,e.g., assuming a gain of full-width half-maximum (FWHM) or greater.

In the MRBR embodiments of FIGS. 1-4, the reflectivity peaks have asubstantially uniform reflectivity profile (envelope), i.e. thereflectivities of the peaks (and thus of the reflectivity bands) aresubstantially the same, for at least a plurality of contiguousreflectivity bands. However, the reflectivity trough minima may varywith wavelength over the wavelength range of interest, in someembodiments, as illustrated in FIGS. 2A, 3A, and 4A, meaning that theenvelope function or profile for the reflectivity trough minima varieswith wavelength. For example, the profile for the reflectivity peaksshown in FIGS. 2A, 3A, 4A is substantially uniform over wavelength,while the profile for the reflectivity trough minima vary overwavelength in accordance with some function.

As will be appreciated, the MRBR of the present invention may have avariety of uses, including wavelength monitoring, selecting, andlocking, as described below. In an alternative embodiment, the MRBR ofthe present invention may be designed with a reflectivity band profilewhich is substantially constant, but in which the reflectivity bandshave a reflectivity not necessarily above any lasing threshold. Such anMRBR may be employed for wavelength monitoring and locking purposes, asdescribed below. When the reflectivity bands of interest have areflectivity above the lasing threshold reflectivity, the MRBR may alsobe employed as one of the mirrors of the laser cavity itself, asdescribed below.

Multiple Reflectivity Band Reflector with Varying Reflectivity BandProfile

In an alternative embodiment, there is provided a reflector with areflectivity band profile that varies as a function of wavelength in aknown manner over a given wavelength range. For example, in anembodiment, the reflectivity band profile may vary monotonically over atuning range, so that each reflectivity band has a unique reflectivitypeak within the tuning range. In such an embodiment, the reflectivitybands need not all be above the lasing threshold reflectivity. Forexample, the reflectivity bands may have peak reflectances from 70% downto 10%, decreasing in appreciable increments (e.g., 2%, 5%, 10%) fromband to band. Such a varying profile reflector may be utilized for laserwavelength monitoring of wavelengths within a given wavelength range ofinterest (e.g., a tuning range of a tunable laser). In otherembodiments, the trough minima (tranmissivity peaks) may be used insteadof the reflectivity peaks. Moreover, because each reflectivity peak (ortrough minima/transmissivity peak, depending on the embodiment) of theMRBR within the wavelength range of interest has a unique reflectivity(at least with respect to its neighboring reflectivity peaks/troughminima), the current lasing wavelength can be determined withoutcounting, as described in further detail below.

In an etalon, two flat, partially reflective parallel reflectors aretypically separated by a parallel spacer. This gives rise to aFabry-Perot interferometer type effect in which the interference ofmultiple beams in the reflective cavity results in constructive anddestructive interference at certain wavelengths. The condition forconstructive interference is that the light forms a standing wavebetween the two mirrors, i.e. the optical distance between the twomirrors must equal an integral number of half wavelengths of theincident light. There will be transmission (a transmissivity peak orreflectivity trough minima) where there is constructive interference,and reflection otherwise. Thus, the interference in the cavity givesrise to a series of equally-spaced transmission/reflection peaks. Thedistance (in wavelength) between adjacent peaks is known as the freespectral range (FSR) or channel separation. The FSR is a function of thephysical mirror separation, i.e. the distance between the two reflectorsof the etalon.

In an embodiment, a varying reflectivity band profile reflector inaccordance with the invention is provided by a modified etalon typereflector having a pair of parallel reflectors, at least one of whichhas a reflectance that varies over the wavelength range of interest(e.g., the tuning range). The reflectors are separated by a spacer layeror air gap of a given thickness (cavity distance). The cavity distancebetween the reflectors determines the FSR and thus the channel spacing,and the combined reflectance characteristic of the reflectors provide anenvelope function for the reflectivity and/or transmissivity peaks.Preferably, the reflectors are substantially symmetrical and havesimilar reflectivity profiles over the tuning range, preferably amonotonically varying reflectance over the tuning range. Thus, a varyingreflectivity band profile reflector in accordance with an embodiment ofthe present invention will have a series of reflectivity bands withinthe tuning range, spaced in accordance with the distance between thereflectors, where the reflectivity peaks of these bands monotonicallyvary over this tuning range.

In an embodiment, the two reflectors of the varying reflectivity bandprofile reflector are quarter-wave stacks, e.g. dielectric DBR typestacks, separated by an intervening spacer layer. In an embodiment, asuitably adapted MRBR may of the present invention may be designedhaving a first quarter-wave reflector section, an intervening spacerlayer, and a second quarter-wave reflector section, where the thicknessof the spacer layer is selected to achieve the desired FSR, and thelayer thicknesses, materials, and number of mirror pairs of the tworeflectors are selected to achieve an overall reflectance envelope thatvaries in a desired manner (e.g., monotonically) over the a given tuningrange. For example, the thickness of the qw layers of a qw stack may beselected based on a given reference wavelength to have a substantiallyhigh and constant reflectance over a wavelength range encompassing thereference wavelength, but which gradually tapers off at the end of thiswavelength range. The reference wavelength may be selected so that thisgradual tapering section occurs over the tuning range. For example, fora tuning range around 1550 nm (e.g., from 1540 to 1580 nm), a referencewavelength of about 800 nm may be selected for a given material system,to result in two qw mirrors each having substantially high and constantreflectance at 800 nm, but already gradually decreasing in the 1540-1580nm range. The combined effect of such a varying reflectivity profile forthe two reflectors of the etalon will be to bound or form amonotonically decreasing envelope around the reflectivity bands in the1540-1580 nm range.

Referring now to FIG. 5A, there is shown a plot of the reflectanceversus wavelength characteristics for an exemplary MRBR with varyingreflectivity band profile in accordance with the present invention anddiagrammed in FIG. 5B. For FIG. 5A the reflectance is determined withunpolarized light impacting the MRBR perpendicularly through air. TheMRBR reflectance characteristics shown in FIG. 5A may be achieved byemploying an MRBR having a layer structure either substantially similarto or based on/derived from the structure shown in FIG. 5B and definedby the following formula:

(AB)¹⁰D(BA)¹⁰  (4)

where the layer symbols in the formula indicate the following materialsand thicknesses:

Symbol Material Thickness A Al₂O₃ 1.5000 qw B TiO₂ 1.5000 qw D Si 6000.0qw

The reference wavelength is 795 nm. By performing the thicknesscalculation, the following thicknesses are obtained:

Symbol Material Thickness A Al₂O₃ 184.03 nm B TiO₂ 129.62 nm D Si 350.74μm

This MRBR may be fabricated with air on both sides, because the Si layer(D) is thick and strong enough to support the MRBR without an externalsubstrate. As can be seen in FIG. 5A, over a wavelength range of 1540 to1580 nm, the reflectivity bands (which are spaced substantiallyuniformly about 1 nm apart) monotonically decrease in reflectivity fromabout 70% to less than 10%. Layer D is a thick layer that separates thesymmetric qw reflectors (AB)¹⁰ and (BA)¹⁰, thus giving rise to theetalon type effect and its associated reflectance characteristic. Inparticular, the thickness of layer D controls the peak separation orFSR. Changing the thickness of layer D therefore changes the channelspacing, and thus may be used to achieve a narrower or wider channelspacing, depending on the application. In an embodiment, the MRBRcomprises two substantially symmetric reflectors, which may bequarter-wave stacks, separated by a distance, e.g. the thickness oflayer D, which may be a material such as silicon, or air in otherembodiments.

As will be appreciated, the MRBR with varying reflectivity band profileof the present invention may have a variety of applications, includingwavelength monitoring, selecting, and locking, as described below.

Single Lasing-Reflectivity Peak Reflector

In an alternative embodiment, a reflector may be designed with a single,narrow, high-reflectivity band which is above the lasing thresholdreflectivity. This single reflectivity band reflector may be referred toas an SRBR, or as a single lasing-reflectivity peak reflector. Like theMRBR, the SRBR is a distributed dielectric multilayer stack reflector.All other reflectivities outside the single reflectivity band, includingother peaks and trough minima, are below this threshold. In anembodiment, this band is narrow and covers a single defined wavelengthof a defined ITU grid set of wavelengths (e.g., around 1.55 μm). Thelasing threshold reflectivity is the reflectivity required for one ofthe mirrors of a VCSEL or VECSEL employing the SRBR as one of itsmirrors, i.e. the reflectivity required of the mirror when it forms partof the laser cavity of a VCSEL, e.g. at least 99.5%. Outside the singlereflectivity band, all other reflectivities of the reflector are lessthan the lasing threshold, preferably less than some second thresholdbelow the first one, e.g. less than 97%.

Referring now to FIGS. 6A-C, there are shown plots of the reflectanceversus wavelength characteristics for an exemplary SRBR, in accordancewith the present invention and diagrammed in FIG. 6D. For FIGS. 6A-C thereflectance is determined with unpolarized light impacting the MRBRperpendicularly through air. The reflectance characteristics shown inFIGS. 6A-C may be achieved by employing an SRBR having a layer structureeither substantially similar to or based on/derived from the structureshown in FIG. 6D and defined by the following formula:

DC(AF)⁴⁰(E)¹⁰⁰(AF)¹⁰⁰G  (5)

where the layer symbols in the formula indicate the following materialsand thicknesses:

Symbol Material Thickness A Al₂O₃ 1.0000 qw C SiO₂ 0.7500 qw D Si 0.7500qw E Si 1.0000 qw F SiO₂ 0.7500 qw G Al 0.0100 qw

The reference wavelength is 1800 m. By performing the thicknesscalculation, the following thicknesses are obtained:

Symbol Material Thickness A Al₂O₃ 277.78 nm C SiO₂ 235.36 nm D Si 99.26nm E Si 132.35 nm F SiO₂ 235.36 nm G Al 1.96 nm

In one embodiment, this SRBR is fabricated on a suitable substrate, suchas glass or InP. As can be seen, e.g. in FIG. 6C, a single reflectivityband is achieved having a peak reflectivity at approximately 1515.8 nm,which is very narrow (i.e., its peak is about 99.5% reflectivity and itcovers, at 99% reflectivity, about 0.02 nm in wavelength, i.e. Δλ₉₉=0.02nm). All other reflectivities of any other peaks (and therefore alltrough minima) of the SRBR of FIG. 6 are less than 97%, i.e. less than asecond threshold (97%) which is itself less than the lasing reflectivitythreshold or reflectivity threshold (e.g., 99%) for the primaryreflectivity band. In this case, if 99% is taken to be the lasingthreshold reflectivity, then Δλ₉₉=Δλ_(LT)=0.02 nm, and this mirror has asingle, narrow reflectivity band above the lasing threshold, i.e. alasing threshold reflectivity band encompassing a single with thatreflectivity peak. In this case, the SRBR has a lasing thresholdreflectivity band of about 0.02 nm in width, with a reflectivity peakwavelength of about 1515.8 nm. Lasing is thus possible at wavelengthsapproximately within this narrow lasing threshold reflectivity band, ifother conditions are met (e.g., the laser is adequately powered, thelasing threshold reflectivity band includes a cavity mode, etc.). Inthis case, the reflectivity band can also be described as having a widthof 0.02 nm at a reflectivity 0.5% below its peak reflectivity (99% isabout 0.5% below 99.5%); or a width of far less than 1 nm at areflectivity a 3% below its peak reflectivity (i.e., at about 96.5%).

Referring now to FIGS. 7A-B, there are shown plots of the reflectanceversus wavelength characteristics for an exemplary SRBR, in accordancewith the present invention. For FIGS. 7A-B the reflectance is determinedwith unpolarized light impacting the MRBR perpendicularly through air.The reflectance characteristics shown in FIGS. 7A-B may be achieved byemploying an SRBR having a layer structure either substantially similarto or based on/derived from the structure shown in FIG. 6D and definedby Formula (6), though different from the layer structure that resultsin the reflectance characteristics of FIGS. 6A-C in that the fewerquarter wavelength silicon layers, layer E, are included. FIG. 6D doesnot indicate the specific number of quarter wavelength silicon layers,layer E, other than indicating that there are more than three. Formula(6) is shown below:

(DC)¹(AF)⁴⁰(E)⁴⁰(AF)¹⁰⁰G  (6)

where the layer symbols in the formula indicate the following materialsand thicknesses:

Symbol Material Thickness A Al₂O₃ 1.0000 qw C SiO₂ 0.7500 qw D Si 0.7500qw E Si 1.0000 qw F SiO₂ 0.7500 qw G Al 0.0010 qw

The reference wavelength is 1800 nm. By performing the thicknesscalculation, the following thicknesses are obtained:

Symbol Material Thickness A Al₂O₃ 277.78 nm C SiO₂ 235.36 nm D Si 99.26nm E Si 132.35 nm F SiO₂ 235.36 nm G Al 1.96 nm

This SRBR may be fabricated on a suitable substrate, such as glass orInP. As can be seen, e.g. in FIG. 7B, a single reflectivity band isachieved with a peak reflectivity wavelength of approximately 1517 nm,which is highly reflective and very narrow (i.e., its peak is about99.4% reflectivity and Δλ₉₉ is about 0.12 nm). All other reflectivitiesof the SRBR are less than 97%.

As will be appreciated, in various embodiments the SRBR of the presentinvention has a variety of uses, including wavelength monitoring,selecting, and locking, as described below.

In an alternative embodiment, the SRBR has one primary reflectivity bandand peak, having a reflectivity which is higher than any otherreflectivity peaks or troughs of the SRBR outside that reflectivityband, but which is not above the lasing threshold reflectivity. Such areflector can be employed, for example, for wavelength monitoringpurposes, but would preferably not be employed as one of the mirrors ofa VCSEL laser cavity.

In the embodiments described above, the SRBR and MRBR of the presentinvention are distributed dielectric multilayer stack structures, i.e.they are reflectors that comprise a plurality of layers of dielectricmaterial arranged in parallel so as to provide the desired reflectivityprofile. In alternative embodiments, other materials may be employed forsome or all of the layers, such as semiconductor materials.

Multiple Reflectivity Band Reflector for Laser Wavelength Monitoring

In an embodiment, the MRBR of the present invention (e.g., theembodiments illustrated in FIGS. 1-5 above) is employed in a laserapparatus for laser wavelength monitoring purposes, in accordance withan embodiment of the invention. As used herein, “monitoring” includesmonitoring, controlling, tuning, selecting, and locking the outputlasing wavelength of a given semiconductor laser with the use of thereflectors of the present invention. The MRBR is part of the lasercavity itself, in some embodiments, or is used outside the cavity formonitoring and locking purposes in other embodiments.

Referring now to FIG. 8, there is shown an embodiment of thesemiconductor laser with MRBR wavelength monitoring of the presentinvention. As illustrated in FIG. 8, a tunable edge-emitting laser 810has an exit mirror 814 (with AR coating) and an HR (high reflectivity,but less than 100%) mirror 816, which define the laser cavity 812. In anembodiment, the laser 810 can emit over a wavelength range coveringseveral ITU wavelengths, under the control of a variable tuningparameter (e.g. temperature, gain, current, etc.). One or moremonitoring photodiodes, e.g. 826 a, 826 b, detect some of the cavitylight 822 passing through the HR mirror 816.

An MRBR 824 covering the tuning range of the laser is used as a filter,to monitor the lasing wavelength and channel. In an embodiment, the MRBR824 is a coating on a light-receiving surface of one of the photodiodes826 b, so that the photodiode 826 b receives light filtered by the MRBR824. In another embodiment, the MRBR 824 is not coated on the photodiode826 b, but is in the path of light 822 from the laser 810 to thephotodiode 826 b. The MRBR 824 preferably has a plurality of narrowreflectivity bands, preferably closely and substantially evenly spaced,over the tuning range of a given laser 810 whose wavelength is to bemonitored, with each discrete target wavelength of the laser 810corresponding to one of the MRBR 824 reflectivity bands' centerwavelength.

Each of the photodiodes 826 a, 826 b is monitored by a correspondingcircuit 828 a, 828 b. The amount of the light 822 transmitted throughthe HR mirror 816 that reaches the photodiodes 826 a, 826 b correspondsto a change in electrical characteristics of the photodiodes 826 a, 826b. In one embodiment, the current flowing through photodiodes 826 a, 826b changes in response to the amount of light 822 received. The circuits828 a, 828 b detect the change in electrical characteristics and reportthat change to a processor 830. In on embodiment, a single circuitperforms the functions of 828 a, 828 b, and 830.

In an embodiment, there is a one-to-one correspondence between the MRBR824 reflectivity bands and the desired selectable (tunable) wavelengthsof the laser 810. In other embodiments, the MRBR 824 can have otherreflectivity bands between the target wavelengths, so long as thelocking algorithm is sophisticated enough to take the “extra”reflectivity bands into account when locking and changing wavelengths.

Monitoring photodiode 826 a is of the type typically used to monitor forpower, in a feedback loop which controls current powering the laser 810(to maintain a constant output power). However, light reaching 826 b isfirst filtered by passing through the MRBR 824, which is coated onto thesurface of the photodiode 826 b. The reflectivity bands of the MRBR 824may have approximately 99% reflectivity, at the center wavelengththereof, and 97% or less reflectivity between the bands (troughs). TheHR mirror 816 may have, for example, 99% reflectivity. Thus, the HRmirror 816 transmits out the “back side” of the laser 810 about 1% ofthe light lasing in the cavity 812. This 1% light 822 impinges on bothphotodiode 826 a and the MRBR 824. Of the light reaching the coatedphotodiode 826 b, the light is either 99% reflected by a bandreflectance, or reflected to a lesser degree if the laser 810 is offband. In another embodiment (e.g., in a preferred embodiment for theembodiment described with reference to FIG. 5A), the photodiode 826 b isnot coated, but the path of light 822 is through the MRBR 824. If thelaser 810 is off band, more light reaches photodiode 826 b, than is thecase when the laser 810 is on band (locked) because the reflectance ofthe MRBR 824 decreases as the wavelength varies from the on bandfrequency for small amounts of variance.

Thus, when the laser 810 is properly tuned to emit at one of the desiredwavelengths (e.g., one of the ITU wavelengths), only about 1% of thelight 822 impinging on the MRBR 824 will be transmitted and reachphotodiode 826 b. If the wavelength starts to drift out of thereflectivity band, a larger fraction of the light 822 will be passedthrough the reflector/filter 824. The increase in light detected byphotodiode 826 b can permit the monitoring circuit/algorithm todetermine that wavelength drift has occurred, and to adjust the tuningparameter (e.g., temperature, gain, and/or current) to get back intowavelength lock.

As will be appreciated, non-coated (or unblocked) photodiode 826 a canbe used as a reference photodiode to monitor power, and to provide areference for photodiode 826 b. Photodiode 826 a is preferably disposednext to photodiode 826 b, where possible, e.g. when an edge-emittinglaser 810 is used, because the edge-emitting beam 822 disperses widely.It is not necessary, though, for photodiode 826 a to be disposed next tophotodiode 826 b. The reference photodiode 826 a can be used as areference and to maintain power across channels (power equalizer).Photodiode 826 b, normalized to photodiode 826 a, can be used bysuitable circuitry or algorithm, see for example FIGS. 14-15, programmedinto a digital signal processor (DSP) 830 to determine when wavelengthlock is being lost. This feedback can be used to tune the laser 810 soas to regain wavelength lock. For example, a temperature controller maybe used to change the temperature of the laser interior 818, to adjustthe lasing wavelength. In an embodiment, the temperature controller islocated in the laser interior 818. In another embodiment, thetemperature controller is proximate to the laser 810.

In an embodiment, a sufficiently sophisticated algorithm (e.g., analgorithm run by a microprocessor or DSP 830 that controls currentinjected into the gain region of the laser 810) may be employed todistinguish power decreases due to aging from changes in light detecteddue to wavelength drift. For example, if the laser 810 is at a properwavelength (on band) then most of the light 822 is reflected by themirror because it is in the reflectivity band. However, if thewavelength drifts, the light detected by photodiode 826 b will increase.If the laser 810 loses power, however, due to aging, then the photodiode826 b will detect a gradual decrease in light. By contrast, loss ofwavelength lock increases the light detected by photodiode 826 b becauseless is reflected by the MRBR 824. Thus, if the wavelength is lockedinto a given channel, a measured increase in light means wavelengthdrift, and a decrease in light means aging-related power decreases. Inan embodiment, this technique may be used even in the absence ofphotodiode 826 a. In embodiments employing a separate, power-monitoringreference photodiode 826 a, however, the algorithm can take this intoaccount, by normalizing the measurement of photodiode 826 b to those ofphotodiode 826 a.

In alternative embodiments, other techniques, e.g. current injection orother techniques for changing the gain (and thus the wavelength), may beemployed to tune the laser's wavelength. In a VECSEL (see FIGS. 10A-B)the effective length of the cavity can be modified resulting in a changein the wavelength of the laser output.

In an embodiment, as described above, the MRBR 824 is physicallyseparated from the HR mirror 816. It may be stand-alone or part of (acoating on) the photodiode 826 b. In an embodiment, the MRBR 824 may betilted at a slight angle (“off angle”) to minimize reflection(interference) back into the laser cavity 812.

In an alternative embodiment, only photodiode 826 b is employed;photodiode 826 a is absent. However, this approach may make it morecomplicated or difficult to monitor power independently of monitoringfor wavelength lock. In another embodiment, for example where onlyphotodiode 826 b is employed, the MRBR 824 is not coated on thephotodiode 826 b, but is instead part of the HR mirror 816 (or coatedthereon). Alternatively, other tapping techniques may be employed, e.g.a tap coupler 910, as illustrated in FIG. 9. In the system shown in FIG.9, the wavelength locker 916 contains the photodiodes 826 a, 826 b, anMRBR coating photodiode 826 b, and suitable circuitry to providefeedback for wavelength locking to the temperature controller 918.

In some applications, for higher frequency or more closely spacedtunable wavelengths, the reflectivity bands of the MRBR 824 need to beextremely close together. One way to achieve this is to design theappropriate MRBR layer structure (typically, a larger number of layers)to provide more closely-spaced reflectivity bands. However, it may beundesirable or impossible to fabricate a thick or complex enough layerstack structure to achieve very close reflectivity band spacing. Analternative technique is to combine an MRBR 824 having a smaller numberof bands, in combination with an air-gap/etalon effect with the HRmirror 816. That is, MRBR 824 combines with HR mirror 816 and theair-gap therebetween, to form an etalon reflector which itself has aseries of reflectivity bands. In this embodiment, the MRBR 824 isparallel to the HR mirror 816 and is separated by a small air distancesufficient to narrow the band spacing. Alternatively, instead of MRBR824, a standard DBR may be employed instead, to combine with HR mirror816 and an air gap to provide an etalon reflector with the desiredspaced reflectivity bands.

In other embodiments, VCSELs (including VECSELs) may be employed insteadof edge-emitting lasers. In such embodiments, the MRBR 824 may be usedto filter tapped laser light before it reaches photodiode 826 b.

In alternative embodiments (as described in further detail below), theMRBR may serve as one of the two cavity mirrors of a VCSEL (i.e., eitherthe top or bottom mirror), or, it may serve as the external mirror of aVECSEL. In these cases, it can be more important that the reflectivitybands have a substantially constant reflectivity profile, and that isalso above the lasing threshold reflectivity. This is because powershould not change too much from selected wavelength to wavelength. Itmay be more expensive and/or difficult to fabricate sufficientlyconstant reflectivity band profile MRBRs, than to fabricate MRBRs withmore variation in reflectivity from band to band. However, the lattertype MRBR, with more unevenness in the band reflectivity from wavelengthto wavelength, which may be cheaper and easier to fabricate, may besufficient for use as an external wavelength monitor/locker, as shown inFIG. 8 or 9.

When the MRBR is not used as one of the cavity mirrors of a VCSEL, it ismore important that the reflectivity bands be sufficiently narrow andprecisely spaced, than that they have uniform reflectivity from band toband, or even that the reflectivity of the bands be above the lasingthreshold reflectivity mentioned above. So long as there is areflectivity band covering each target wavelength, locking photodiode826 b will detect an increase in detected intensity when lock starts tobe lost. In particular, photodiode 826 b will see a change from whateverthe detected intensity is while in lock, to a higher amount (forwavelength shift) or lower amount (for aging-cased decrease in power).This happens even if there is an uneven reflectivity peak profile.Alternatively, the rate of change may be measured; and/or thereflectivity peak profile may be known and taken into account by asuitable algorithm.

In an embodiment, a coarse tuning parameter (e.g. temperature, current)is used to adjust the wavelength of the laser. By monitoring the lasingwavelength with the MRBR of the present invention, a suitable algorithmcan be employed to lock onto any of the center wavelengths of the targetreflectivity band. For example, a given MRBR may have reflectivity bandscentered at several substantially evenly-spaced wavelengths, λ₁, λ₂, . .. λ_(N). Initial calibration of a given laser can be done to correlate agiven tuning parameter with a given wavelength window associated withone of the MRBR's reflectivity band wavelengths. Where an MRBR as shownin FIGS. 1-4 is employed, with a substantially unchanging reflectivityband profile, the coarse tuning parameter is expected to achieve awavelength near the center wavelength of the corresponding reflectivityband. The tuning parameter can be varied under the control of a suitablealgorithm, to reach the center wavelength of the reflectivity band(e.g., by minimizing the light detected by photodiode 826 b, becausemaximum reflectivity, and thus minimum transmission, occurs when thelaser is lasing at the center wavelength of a given reflectivity band asdescribed in FIG. 14). Once it is known, e.g. by calibration, that thelaser is locked onto a given wavelength, the coarse tuning parameter(s)can be varied to quickly select another wavelength corresponding to thecenter wavelength of another reflectivity band, see FIG. 15.

Additionally, because the difference in light measured will besubstantially greater in the troughs than at the peaks, simple integercounting can be used to know exactly which channel the laser isoperating at, and, e.g., to quickly change channels (e.g., from channel2 to channel 7, by counting 5 pulses). I.e., by use of a suitablealgorithm, transitions over reflectivity bands can be counted, toprecisely select another discrete wavelength. Such counting is shown inFIG. 15.

As with the embodiments described above with reference to FIGS. 8-9, theMRBR may be either part of the laser 810 or part of the monitoringmodule, e.g. coated on photodiode 826 b, between photodiode 826 b andthe HR mirror 816, or it may form the HR mirror 816 itself (assuming,for the last embodiment, that it has sufficient reflectivity to permitlasing to occur).

Although an MRBR may be employed having a substantially constantreflectivity band profile for wavelength monitoring (whether or not thereflectivity bands have a reflectivity above or below a lasing thresholdreflectivity), an MRBR may be employed which has a substantially varyingreflectivity profile (i.e., a varying peak profile and/or troughprofile) over the wavelength range of interest (e.g., tuning range). Forexample, an MRBR may be employed having a plurality of reflectivitypeaks which have a monotonically varying reflectivity band peak profileover the tuning range. Alternatively, the trough minima may be utilizedfor monitoring/locking, where the trough minima have a known, varyingtrough profile. In the former case, each reflectivity peak has a uniquereflectivity with respect to its neighbors, thereby establishing aunique reference for each discrete wavelength and thus permittingdetermination of the current lasing wavelength without counting.

The MRBR of FIGS. 5A-B, for example, may be employed as the MRBR of thelaser system of FIG. 8, or of a similar system not having photodiode 826a. In this embodiment, the MRBR has a monotonically varying amplitudeprofile over the tuning range of the laser. The MRBR has bothreflectivity peaks and troughs (corresponding to transmissivity troughsand peaks, respectively). Either or both the reflectivity peaks andtrough minima may be used to lock onto a selected wavelength, and alsoto determine which wavelength the laser is currently locked onto. In anembodiment, due to the unique reflectivity of each (or adjacent)reflectivity bands in the tuning range, the current lasing wavelengthcan be determined without counting. FIG. 14 shows this method of lockingonto a wavelength of either a peak or a trough minima.

In alternative embodiments, an MRBR 824 may be employed which has avarying reflectivity trough profile as well, or instead of, a varyingpeak profile. For example, the MRBRs of FIGS. 2-4 have varying troughprofiles. In this case, the reflectivity trough minima (e.g.,transmissivity peak) may be used to achieve lock. The reflectivitytrough minima may be much narrower than are the reflectivity band peaks,and thus may provide more precise wavelength monitoring and locking.

In further embodiments, the peak (or trough) profile need not varymonotonically. As long as each neighboring (adjacent) peak (or troughminima) varies from its neighbors in a known way, and to a great enoughdegree to permit differences in measured intensity to be adequatelydistinguished, an appropriate locking algorithm can determine thecurrent lasing wavelength, without counting in some embodiments, andprovide for wavelength locking as described hereinabove.

Thus, in some embodiments, an MRBR is employed which has a varyingreflectivity profile (i.e., a varying peak profile and/or troughprofile), in which neighboring reflectivity peaks (or trough minima,depending on which are used for locking) vary from each other inreflectivity substantially enough, and in a known way, so that anappropriate locking algorithm can determine the current lasingwavelength without counting. That is, the reflectivity profile is anon-constant function that varies sufficiently to permit the differencesin reflectivity of the peaks (or trough minima) to provide usefulinformation to the locking algorithm. The reflectivity profile may varymonotonically, for example substantially linearly, over thetuning/monitoring range, as in the MRBR of FIGS. 5A-B. As noted above,this MRBR has over a wavelength range of 1540 to 1580 nm, reflectivityband peaks spaced substantially uniformly about 1 nm apart whichmonotonically and substantially linearly decrease in reflectivity fromabout 70% to less than 10%. In particular, as illustrated in FIG. 5A,this MRBR has at least at least fourteen reflectivity (wavelength) bandsover a certain range, where the reflectivity peaks vary in reflectivityby more than 20%, but each deviates by less than 5% reflectivity from alinear relationship of length of wavelength to percentage ofreflectance.

In other embodiments, other MRBRs having substantially varyingreflectivity profiles may be employed. For example, other structures maybe employed for the MRBR to give rise to a substantially linearreflectivity profile in which each reflectivity peak within the tuningrange varies in reflectivity by at least a minimum threshold amount(e.g., 5% or 10%) from its neighboring peaks, and deviates by less thana maximum tolerance (e.g., 5%) from a linear relationship of wavelengthto percentage of reflectance. In other embodiments, the reflectivityprofile may be monotonic but non-linear, where the peaks deviate fromthis function by less than some maximum tolerance. Or, the reflectivityprofile function may be non-monotonic, such as the shape of the troughprofile of the MRBR shown in FIG. 3A.

In an embodiment, the reflectivity peaks each correspond to one of thetarget lasing wavelengths of laser 810. In one embodiment, the peak ofeach reflectivity band (i.e., its center wavelength) corresponds to thetarget wavelength. In this embodiment, if an MRBR having a varyingreflectivity band profile is employed, then each selectable wavelengthcorresponds to a unique reflectivity (peak or trough minimum), and thusto a unique detected intensity upon lock (normalizing for age- orpower-related changes in intensity). Thus, a locking algorithm may beemployed, which can keep the laser locked onto the desired wavelength(by adjusting the laser's tuning parameter to prevent wavelength drift),and which can also determine which wavelength the laser is currentlylocked onto, without the necessity of the counting described above forMRBRs having substantially constant reflectivity band profiles.

Wavelength drift in either direction will result in an increase in lightdetected by photodiode 826 b. Therefore, a sufficiently sophisticatedlocking algorithm is preferably employed, which can determine in whichdirection the drift is occurring and correct it. For example, historicaldata or other data may be consulted to make a best guess as to whichdirection the wavelength is drifting, in cases of increased lightdetection by photodiode 826 b. Or, an arbitrary guess may be made. Ineither case, the algorithm adjusts the laser wavelength, hopefully inthe appropriate direction, through the use of negative feedback, untillock is regained. If the adjustment exacerbates wavelength drift, it canbe assumed that the wrong assumption was made and corrections in theappropriate direction can be made. See FIG. 14.

In another embodiment, a specified reflectivity point on the “side” ofthe band, less than the maximum reflectivity, is selected to correspondto the target wavelength. In such an embodiment, the direction ofwavelength drift is unambiguous. In this case, the locking algorithmconstantly controls the tuning parameter to maintain the specified pointon the corresponding reflectivity band. In an embodiment, a referencephotodiode 826 a may be used to correct for changes in intensity causedby factors other than wavelength drift, e.g. aging. Another advantage ofthis approach is that it is not as critical to design and fabricate anMRBR 824 where each band has its center wavelength precisely matchedwith the target wavelengths. Instead, it is sufficient that there be atleast one reflectivity band (or side) per target wavelength, with aknown profile, so that the exact target wavelength can be correlatedwith a given percentage reflectivity of the peak reflectivity of thecorresponding band. For example, this is indicated by the exemplarysolid dots on the sides of some of the reflectivity bands of FIG. 5A.(In an alternative embodiment, multiple target wavelengths cancorrespond to different reflectivities on the same side of a givenreflectivity band.) In such an embodiment, the monitoring algorithm isstill able to quickly determine the current lasing wavelength withoutcounting, by finding the peak of the current reflectivity band (e.g.,during an initialization period) and correlating its reflectivity (orits proxy, the lasing intensity measured by photodiode 826 b) with theappropriate band. Thereafter, the algorithm can back off of the centerof the band to find the appropriate place on its side that correspondswith the target wavelength desired.

The varying-profile MRBR 824 may be employed either in conjunction withpower-monitoring photodiode 826 a, or in an embodiment in which onlyphotodiode 826 b is employed. Preferably, photodiode 826 a is used tostabilize power, and photodiode 826 b (normalized to photodiode 826 a)is used to detect wavelength.

VECSEL Employing Single Lasing-Reflectivity Peak Reflector

The SRBR of FIG. 7 is a distributed (multilayer stack) reflector with astructure that gives rise to a single, narrow, high-reflectivity peak orband, somewhere within a given wavelength range of interest, asdescribed above. In an embodiment, the single reflectivity band is anarrow lasing threshold reflectivity band (having reflectivity above alasing threshold reflectivity of, e.g., 99%) covering a singlewavelength of an ITU grid set of wavelengths (e.g., one of those around1.55 μm). I.e., Δλ₉₉ for the SRBR covers one of the ITU gridwavelengths. The high reflectivity figure corresponds to the lasingthreshold reflectivity for a VECSEL (e.g., typically 99% to 99.5%), ator near the gain spectrum maximum. Outside the single high-reflectivityband all other reflectivities of the SRBR in the wavelength range ofinterest are less than some second reflectivity threshold below thelasing threshold reflectivity, e.g. less than 97%.

For a given set of parameters, Δλ₉₉ encompasses a single wavelength ofthe ITU grid, but the parameters can be changed to adjust Δλ₉₉ to coveranother ITU grid wavelength. For example, for a given distributeddielectric multilayer stack structure, changing the refractive index ofone or more layers of the multilayer stack can shift the wavelengthpeak, i.e. the Δλ₉₉. This may be done by changing the temperature of theSRBR, although this can be a slower way to change the reflectancespectrum of the SRBR than other techniques, such as piezoelectrictechniques. For example, some of the layers of an SRBR can consist ofpiezoelectric or electrooptic material, in which case a voltage sourcecan be used to provide a voltage (electric field) across thepiezoelectric layers of the SRBR to vary the refractive index, which canshift the reflectivity spectrum of the SRBR.

An SRBR may be utilized for a variety of applications, including use forwavelength locking onto a fixed wavelength or for use as the tuningelement in a tunable VECSEL.

In an embodiment shown in FIG. 10A, an SRBR 1018 is used as the thirdreflector of a VECSEL 1010. Its reflectivity spectrum may be changed bychanging its refractive index (or of some of its layers), for example toprovide a tunable laser system. Its refractive index may be changed by,for example, wavelength tuning device 1030. In an embodiment, wavelengthtuning device 1030 may be an optical pump laser, a voltage source, or aheating source. An optical pump laser, for example, may work well if theSRBR has semiconductor layers for some or all of its layers, instead ofonly dielectric layers. However, it may be difficult, impractical, orotherwise undesirable to fabricate an SRBR using semiconductor layers.Thus, in an embodiment, device 1030 provides a means for changing otherparameters of SRBR 1018 such as temperature or voltage. For example,some of the layers of SRBR 1018 can consist of piezoelectric orelectrooptic material, in which case device 1030 is a voltage sourcewhich provides a voltage (electric field) across the piezoelectriclayers of the SRBR to vary the refractive index, which can shift thereflectivity spectrum, and thus the wavelength covered by the singlereflectivity peak, of the SRBR.

As shown in FIG. 10, a VECSEL 1010 has a two-section cavity 1012, withbottom mirror 1014, exit mirror 1016, and third mirror 1018 formed bythe SRBR. The output 1021 of the VECSEL 1010 is through the exit mirror1021. The presence of output at both angles of the two-section externalcavity can decrease efficiency compared to the implementation of FIG.10B. The VECSEL 1010 comprises an active region 1020, and a power sourcefor electrically or optically pumping the active region. For an OPlaser, the power source may be an external pumping laser (not shown);for an EP laser, the power source may be a current and/or voltagesource. In either case, the power source is functionally coupled to theactive region when the pumping energy may be applied thereto by thepower source.

A given laser, such as VECSEL 1010, has a certain gain spectrum. Thegain spectrum typically has a maximum at a particular wavelength. Thegain spectrum, including its maximum, can be shifted with respect towavelength, e.g. by changing the temperature or pumping energy. Also,because the mirrors are not 100% reflective, a loss is introduced.Lasing is possible where there is sufficient reflectivity (i.e., wherethe gain exceeds the loss, i.e. where there is a net gain) and wherethere is a cavity mode. Typically a broad reflectivity spectrum isprovided by DBR cavity mirrors, so that there is a small loss over awide wavelength range which usually includes the gain spectrum maximum.There can be several cavity modes within the wavelength range wherelasing is possible, i.e. where there is net gain. Typically, the modeclosest to the net gain maximum will be selected and will win outthrough mode competition, although multi-mode operation and mode hoppingcan occur if too many cavity modes exist close together, close to thenet gain maximum.

In the present invention, the SRBR provides a narrow reflectivity bandabove the lasing threshold, so that the laser has net loss (loss exceedsgain) for all but a narrow lasing threshold reflectivity band covered bythe peak of the single reflectivity band. Thus there is a smallerwavelength range over which lasing is possible, namely the narrow lasingthreshold reflectivity band encompassing the single peak of the SRBR,assuming it intersects the gain spectrum at a high enough gain (e.g., ator near the gain maximum) so that the net gain is high enough to permitlasing to occur. In addition, lasing can only occur if the narrow lasingwavelength band of the single peak includes a cavity mode. The VECSEL1010 will provide single-mode lasing at a cavity mode wavelength withinthe single lasing threshold reflectivity band of the SRBR 1018 if itintersects the gain spectrum at a high enough gain. Because the lasingthreshold reflectivity band is very narrow, there will likely be onlyone cavity mode in the lasing threshold reflectivity band. Also, becauseof the narrowness of the lasing threshold reflectivity band, the lasingwavelength will be approximately the center or peak wavelength of thesingle lasing threshold reflectivity band.

As an example, if the single lasing threshold reflectivity band of SRBR1018 covers an ITU grid wavelength and a cavity mode wavelength, and ifthe lasing threshold reflectivity band intersects the gain spectrum nearthe gain maximum, there can be a large net gain and stable lasing at thecavity mode wavelength, which is approximately the desired ITU gridwavelength. For comparison purposes, if the third mirror 1018 insteadhad an MRBR with many closely-spaced reflectivity bands, changes in thegain spectrum could select one or more of the reflectivity bands forlasing; and mode hopping could occur in some applications. However, inan embodiment illustrated in FIG. 10A, third mirror 1018 comprises anSRBR, which, because of the single peak, can have superior mode-hoppingrejection characteristics. Also, the gain can be adjusted withoutaffecting the lasing wavelength, because lasing is only possible at thesingle peak wavelength.

Thus, by utilizing an SRBR as one of the mirrors in the three-mirror,two-section cavity of a VECSEL, the reflectivity peak wavelength of theSRBR helps the laser lock onto that wavelength, and the gain of theactive region may be adjusted to some degree independently of the lasingwavelength.

The reflectivity peak of the SRBR can either be fixed or it can betunable. In the latter case, an MRBR or other type of filter may beemployed along with a wavelength locking circuit to tune the SRBR peakto the desired wavelength. Power monitoring elements and circuitry canbe used to adjust the gain independently.

It is often possible to independently tune or adjust gain and the cavitymodes. For example, changing the pumping power shifts the gain spectrum,but only has relatively minor, secondary effects on the cavity modes. Bycontrast, more changing the index of refraction of various layers of thecavity can shift the cavity mode. This may be done by adjusting thetemperature of the laser. Thus, for example, assume a VECSEL or VCSELemploying an SRBR, which has a fixed wavelength single peak at thedesired ITU grid lasing wavelength. When first using or calibrating thelaser, the pumping power and temperature are set at initial levels. Ifthere is no lasing, it can be assumed that the single reflectivity peakdoes not intersect a cavity mode, i.e. the cavity mode is slightly“off”. Thus, the temperature can be adjusted by a controller undercontrol of a suitable algorithm, to adjust the cavity modes, untillasing occurs, at which point the cavity mode has been calibrated to besufficiently close to the single reflectivity band and the desiredlasing wavelength. Preferably, the temperature (and thus wavelength ofthe cavity mode) is adjusted until the output power is maximized; atthis point, the cavity mode is precisely located at the peak of thesingle reflectivity band. Then, if the power needs to be higher orlower, the pumping power can be adjusted to adjust the gain spectrum. Ifonly minor power adjustments are needed, the cavity mode may not changeappreciably. However, if adjusting the gain spectrum to change theoutput power results in an appreciable shift in the cavity mode, thetemperature can again be adjusted to counter the second-ordercavity-mode shifting effect of the gain adjustment.

In an embodiment, SRBR is tunable to provide a tunable VECSEL. In thiscase, as illustrated in FIGS. 10A-B, a monitor photodiode 1024 andaccompanying measurement circuit 1026 measure the amount of lighttransmitted through the SRBR 1018. The light 1022 may be passed throughan MRBR or other wavelength-sensitive filter first, as described withreference to the wavelength locking function of FIG. 8. That measurementis then used by a processor 1028 to calculate a change in operation of areflectivity wavelength tuning device 1030 to obtain a particular peakwavelength for SRBR 1018. As discussed with respect to FIG. 8, monitorphotodiode 1024 can include an MRBR coating to assist in segregatingparticular desired wavelengths, or an MRBR or other type of wavelengthfilter placed between reflector 1018 and the monitor photodiode 1024.The same types of feedback algorithms discussed with respect to FIG. 8can also be used to modify SRBR 1018 in response to measurements of themonitor photodiode 1018.

Monitor photodiode 1024 can also be used independent of wavelength toprovide feedback indicative of the laser output power. That feedback canthen be used to determine in part the proper pumping level provided bythe power source coupled to the active region 1020 as disclosed withrespect to FIG. 8. Alternatively, other taps or monitor photodiodes maybe employed to monitor the output intensity to adjust the gain,independent of the lasing wavelength.

Thus, in an embodiment, as shown in FIG. 10A, the effective wavelengthof the single peak of the SRBR 1018 is changed, to tune the lasingwavelength of the VECSEL 1010. In an embodiment, the wavelength tuningdevice 1030 is used to shift the single lasing threshold reflectivityband of SRBR 1018 (and its peak reflectivity wavelength) up or down inwavelength, thereby adjusting the overall lasing wavelength of theVECSEL 1010. As noted, device 1030 may use any suitable technique tochange the reflectivity profile of the SRBR 1018, such as temperature orpiezoelectric techniques.

In another embodiment, the monitor photodiode 1024 is employed, e.g.with a tap as shown in FIG. 9, to monitor the actual lasing wavelength,and to provide feedback to the optical pump 1030 or other adjustmentdevice, to lock onto a desired wavelength. In this embodiment, anexternal MRBR may be employed with the monitor photodiode 1024, asdescribed above, for the wavelength monitoring and locking function. Inyet another embodiment, a separate monitor may be employed to monitorthe power of laser 1030, to ensure that it is supplying the desiredoptical pumping to the SRBR 1018, which is calculated to achieve adesired change in wavelength, under certain conditions. For example, theSRBR 1018 may be temperature controlled so that varying the optical pumppower in a known way changes the wavelength of the single peak in aknown way.

The various embodiments discussed above can also be implemented with thecavity 1012 arranged around the SRBR 1018 with the exit mirror 1016, andthus the output 1021, at one end of the cavity 1012, as shown in FIG.10B.

In an alternative embodiment of the implementation illustrated in FIG.10B, mirror 1018 is an SRBR and the exit mirror is an MRBR so that thelaser only produces desired wavelengths and the particular wavelengthproduced depends upon the parameters (e.g., optical pumping,temperature, or voltage) applied to the SRBR.

In an alternative embodiment, an appropriately designed MRBR may be usedas an SRBR. For example, an MRBR that has sufficiently spread-outreflectivity peaks can function as an SRBR if only one of itsreflectivity peaks lies within the wavelength range of interest, i.e.those wavelengths at which lasing might be desired. Such a reflector maybe referred to as an MRBR-type SRBR, or MRBR-SRBR. For example, thewavelength range of interest may be a set or subset of ITU gridwavelengths for which the gain spectrum of the active region is designedto operate; or it may be the wavelength range which can be covered by asufficiently high gain, e.g. within the range of wavelengths covered bythe FWHM of the gain spectrum, including wavelengths covered when thegain spectrum is shifted by adjusting its power source. In such a case,within the wavelength range of interest, the MRBR provides only a singlepeak; all other reflectivity peaks of the MRBR fall outside thewavelength range of interest. For example, the MRBR has a single peak inthe wavelength range of interest, which intersects the gain spectrum ata sufficiently high gain so that there is net gain; and all other peaksintersect the gain spectrum at a much lower gain so that there is verylow, or negative, net gain, in which case there will be single-modelasing at only the single peak's wavelength. Thus, for such anSRBR/MRBR, within the gain spectrum range of interest, there is only asingle reflectivity peak; and all other reflectivities outside thesingle peak or band, in the wavelength range of interest, are below thisthreshold. That is, within the wavelength range of interest (i.e., thewavelength range covered by the gain spectrum range), there is only asingle reflectivity peak of the MRBR that is above the lasing threshold;all other reflectivities of the MRBR in the spectrum of interest areless than some second threshold below the first one, e.g. less than 97%.

As described below, in alternative embodiments an SRBR may be employedfor wavelength locking with an integrated, monolithic VCSEL, and with aone-section VECSEL.

Wavelength-locked Laser with Multiple Reflectivity Band Reflector

In an embodiment, similar to the configuration shown in FIG. 10A or 10B(where the third mirror 1018 is an MRBR instead of an SRBR), a VECSEL1010 employs a bottom mirror 1014, an exit mirror 1016, and an external“top” reflector 1018 to complete the two-section cavity 1012. One of thethree mirrors of the VECSEL 1010, preferably the external, “top”reflector 1018, is an MRBR in accordance with the present invention,having reflectivity bands which are spaced along target selectablelasing wavelengths (preferably along the ITU grid). The VECSELstructure, including cavity length and phase-matching layers, ispreferably also designed so that cavity modes fall along each of thetarget selectable lasing wavelengths.

If the MRBR is the third mirror 1018, the reflectivities of thereflectivity bands need to be above the lasing threshold reflectivity,e.g. 99.5%. This permits sufficient reflection in the lasing cavity 1012at one of the desired wavelengths. For example, the MRBRs of FIGS. 1-4can be used for this purpose.

The active region 1020 of the VECSEL 1010 can be either electrically oroptically pumped. By adjusting the electrical or optical pumping power,the gain spectrum of the VECSEL 1010 can be shifted. As the gainspectrum shifts, it overlaps with different reflectivity bands of theMRBR, causing discrete “jumping” from one lasing wavelength to another.The bands are preferably far enough apart (e.g., a CWDM type spacing) toprevent mode hopping and to achieve single mode operation. Thus, as longas the coarse gain spectrum tuning of the active region 1020 selectsclose to the desired wavelength range covered by the target reflectivityband, a stable lasing wavelength at the narrow wavelength of thereflectivity band will be achieved. This can avoid or reduce the needfor feedback that is often required in continuously tunable lasers. Inanother embodiment, feedback can be implemented to fine tune to peaks ofthe MRBR 1018, adjusting the MRBR band positioning through opticalpumping, temperature control, or piezoelectric voltage control asdiscussed above.

In other alternative embodiments, either bottom mirror 1014 or exitmirror 1016 may comprise an MRBR. If bottom mirror 1014 is the MRBR,then as for top mirror 1018, the reflectivity bands need to havereflectivity above the lasing threshold reflectivity. If exit mirror1016 is the MRBR, the reflectivity bands cannot have 100% or too highreflectivity (e.g., it should be somewhat smaller than the reflectivityof the bottom mirror 1014, which typically has a reflectivity >99.8%),because some light must be transmitted as output 1021.

Instill further alternative embodiments, two of the mirrors 1014, 1018of the VECSEL 1010 may have MRBRs, which can combine in a “verniereffect” to provide further wavelength selectability.

As described below, in alternative embodiments an MRBR may be employedwith an integrated, monolithic VCSEL, and with a one-section VECSEL.

Integrated Wavelength-locked VCSEL with Multiple or Single ReflectivityBand Reflector

In an embodiment, as illustrated in FIG. 11, an integrated VCSEL employsa bottom mirror 1112 and a top exit mirror 1116 to complete the lasercavity. The structure is supported by a substrate 1110 with the laseroutput 1118 perpendicular to the substrate 1110. One of the two mirrorsof the VCSEL, preferably the bottom reflector 1112, is an MRBR inaccordance with the present invention, having reflectivity bands whichare spaced along target selectable lasing wavelengths (preferably alongthe ITU grid). The VCSEL operates similarly to the wavelength-lockedVECSEL with MRBR as described above. The active region 1114 of the VCSELis preferably electrically pumped and tunable by electrically changingits gain spectrum. In this embodiment, the reflectivity band profile ofthe MRBR (e.g. 1112) is fixed. Target wavelengths are selected bychanging the gain spectrum of the VECSEL, and/or by making any changesin the cavity modes by appropriate temperature tuning. In an alternativeembodiment, the MRBR forms exit mirror 1116, instead of bottom mirror1112.

In another embodiment, an SRBR is used to form either exit mirror 1116or bottom mirror 1112. In this case, the lasing wavelength can be fixed,while permitting gain to be adjusted. Alternatively, if tuning isdesired, bottom mirror 1112, for example, can be an SRBR, suitabletechniques (e.g., piezoelectric) may be employed to change thereflectivity profile of the SRBR, and thus to change the lasingwavelength. In such a case, for example, extra terminals may need to beprovided to permit piezoelectric tuning of the SRBR, independent fromproviding electrical pumping to the gain region 1114.

One-Section External Cavity Wavelength-locked VECSEL with Multiple orSingle Reflectivity Band Reflector

In an alternative embodiment, there is provided a one-section cavity,non-integrated VECSEL, in which the top (exit) mirror is externallymounted above an integrated bottom laser portion having the activeregion and bottom mirror. One of the mirrors, either the top or bottommirror, is either an SRBR or MRBR, to provide for wavelength locking onthe wavelengths associated with reflectivity peak(s) of the SRBR or MRBRof the VECSEL. The VECSEL operates similarly to the wavelength-lockedVECSEL with MRBR or SRBR as described above.

Reflectively Coupled Zigzag Waveguide Device for Wavelength Locking

As noted above, U.S. Pat. No. 5,894,535 and Brian E. Lemoff et al., “ACompact, Low-Cost WDM Transceiver for the LAN,” 2000 Proceedings,50^(th) Electronic Components & Technology Conf. (IEEE 2000) disclose areflectively coupled zigzag waveguide device which is used forwavelength demultiplexing. The '535 patent, for example, discloses adevice including a dielectric waveguide that guides a WDM signal througha zigzag path. The WDM signal contains multiple light signals at severaldiscrete wavelengths of light, e.g. λ₁, λ₂, . . . λ_(N). At particularvertices of the path optical filters selectively transmit and reflectwavelengths of light. Each of the filters has a unique passband centeredon (having a transmissivity maximum at) one of the plurality ofwavelengths λ₁, λ₂, . . . λ_(N). Each filter (relatively) passes lightin the passband centered on its respective wavelength λ_(i), andrelatively reflects light outside this passband. Each filter maytherefore be referred to as a mirror/filter. The zigzag waveguide deviceof the '535 patent outputs, at each of the plurality of mirror/filters,different wavelengths of light, λ₁, λ₂, . . . λ_(N), thus demultiplexingthe WDM signal to extract individual signals at each of the discretewavelengths making up the WDM signal.

In the present invention, a reflectively coupled zigzag waveguide deviceis implemented as a wavelength locker instead of as a demultiplexer.Referring now to FIG. 16, there is shown a zigzag waveguide device-basedwavelength locking apparatus 1600, in accordance with an embodiment ofthe present invention. Apparatus 1600 comprises zigzag waveguide device1630, coupled to fiber 1610 to receive a tap of lasing light from anexternal tunable laser (not shown). Zigzag waveguide device 1630 is azigzag patterned dielectric channel waveguide structure that guides alight signal through a zigzag path. At particular vertices of the pathoptical filters selectively transmit and reflect wavelengths of light.Zigzag waveguide device 1630 may also be referred to as a reflectivelycoupled optical waveguide structure, which is embedded in a substrate.Such a structure is a planar optical device that includes two or moreoptical channel waveguides oriented such that two adjacent waveguidesconverge at a vertex.

The tap of laser light received is at a current actual wavelength λ_(A),which may or may not be substantially equal to the desired or targetlasing wavelength λ_(L). The desired lasing wavelength λ_(L) is aparticular one of a plurality of discrete lasing wavelengths λ₁, λ₂, . .. λ_(N), i.e. L=1, 2, 3, . . . or N. If λ_(A)=λ_(L), laser lock has beenachieved. If not, the laser's lasing wavelength needs to be adjusted tomove λ_(A) in the direction of λ_(L), until they are equal orsufficiently equal (i.e., the difference λ_(A) and λ_(L) is less thansome predetermined threshold difference). The purpose of wavelengthlocking apparatus 1600 is to generate control signals to perform thisadjusting, so as to achieve and/or maintain laser lock on the desiredlasing wavelength λ_(L).

Zigzag waveguide device 1630 comprises a plurality N of opticalfilters/reflectors 1614, each filter 1614 _(i) having a passband at arespective wavelength λ_(i) which is a corresponding one of the Ndiscrete wavelengths λ₁, λ₂, . . . λ_(N). In an embodiment, the filters1614 are substantially identical except that their structures areadapted to provide different passbands. Filter 1614 may be distributeddielectric multilayer stack reflectors, for example.

As noted above, each filter 1614 _(i) (relatively) passes light in thepassband centered on its respective wavelength λ_(i), and relativelyreflects light outside this passband. Further, because the passband isnot ideal, light at exactly the center wavelength is transmitted at ahigher degree than is light not exactly at the center wavelength λ_(i)but still close to wavelength λ_(i) and within the passband. Each filter1614 _(i) therefore produces a respective filtered output signal,related to the intensity of light impinging thereon as well as thewavelength of such light. Light within the passband produces a largeroutput signal the greater its intensity and also the closer it is to thecenter wavelength λ_(i). Apparatus 1600 also comprises a correspondingplurality N of photosensors, e.g. photodiodes, and measuring circuit1618, arranged so that a photodiode and measuring circuit 1618 _(i) ispositioned to received the filtered output of the corresponding filter1614 _(i). In the illustrated embodiment, N=4.

The input optical signal at actual lasing wavelength λ_(A) propagatesthrough a waveguide 1612 of zigzag device 1630 until it reaches a firstfilter 1614 a at a vertex of the zigzag patterned waveguide structure1612. In an embodiment, each vertex of the zigzag waveguide structure1612 has a vertex angle of approximately 12°, but may be at other anglesin other embodiments, e.g. within a range of 3° to 45°. Zigzag waveguidestructure 1612 is formed by a number of dielectric channel waveguides,comprised of dielectric layers embedded in a cladding region 1619 of thesubstrate. The cladding region can be a dielectric layer on thesubstrate or it can be the substrate itself. In either case thewaveguides may be said to be embedded and/or patterned in a substrate.The cladding region 1619 has a cladding refractive index, and thedielectric channel waveguides of structure 1612 have a waveguiderefractive index, which is higher than the cladding refractive indexsuch that light is confined within the waveguide structure. Waveguidestructure 1612 is embedded in the substrate using known means andmethods.

In an embodiment, each waveguide of the zigzag waveguide structure 1612is substantially rectangular in cross section and is approximately 70microns high by 100 microns wide. Other dimensions and shapes can alsobe employed for the waveguides.

The first optical filter 1614 a includes layers with the opticalproperty of transmitting a wavelength band (passband) centered on aparticular wavelength, e.g. λ₁, but reflecting other wavelengths atleast relative to the transmittance of wavelengths within the passband.Thus, the optical filter transmits light in a particular wavelengthrange (passband) out of the waveguide structure and reflects light inother wavelength ranges into the subsequent waveguide.

The reflected light continues to travel through the waveguide until itreaches a full spectrum reflector 1616. Substantially all the light isreflected from the full spectrum reflector 1616 and continues to thenext vertex at which is located second reflector/filter 1614 b, which issimilar to the first reflector 1614 a with a different wavelength band,namely one centered on λ₂. The third reflector 1614 c and fourthreflector 1614 d are also configured to have higher transmittance forlight in their corresponding wavelength bands centered on wavelengths λ₃and λ₄. The light transmitted by each of the reflectors 1614 a-d isdetected by a corresponding photodiode and measuring circuit 1618 a-d.

Wavelength locking apparatus 1600 comprises a wavelength locker circuit1620, which may be, or comprise, a processor. Wavelength locker circuit1620 receives the signals generated by the N photodiode and measuringcircuits 1618. That is, wavelength locker circuit 1620 receives Nsignals, each signal corresponding to the intensity of light within arespective passband. It may also receive the output from a powermonitoring photodiode (not shown), for use as a reference, as describedbelow with reference to FIG. 12. Wavelength locker circuit 1620 ispresumed to have knowledge of the desired lasing wavelength, i.e. itknows which of the possible lasing wavelengths (λ₁, λ₂, . . . λ_(N)) isthe desired wavelength λ_(L).

Wavelength locker circuit 1620 analyzes the signals with a suitablealgorithm and controls the transmitting laser in accordance therewith.In particular, based on the photodiodes' signals, wavelength lockercircuit 1620 determines whether the laser is lasing at the desiredlasing wavelength λ_(L), that is, whether λ_(A)=λ_(L). If not, circuit1620 generates control signals to provide to driver control circuitrycoupled to the tunable laser, to adjust the lasing wavelength of thelaser so as to achieve lock on the desired lasing wavelength λ_(L). Ifλ_(L) is λ₂, for example, then circuit 1620 makes sure that the outputsignal from photodiode and measuring circuit 1618 b is at a maximum.

As an example, if λ₂ is selected to be the desired lasing wavelength,then circuit 1620 (or some other circuit) sends control signalspredetermined to achieve this lasing wavelength to the laser. This mightnot achieve the exact wavelength λ₂, however, due to slight mismatchesin calibration or aging. For example, the laser may be lasing at actualwavelength λ_(A) which is close to, but not exactly equal to, λ₂. Inthis case, the outputs of photodiode and measuring circuits 1618 a, 1618c, and 1618 d (corresponding to wavelength passbands at λ₁, λ₃, and λ₄,respectively) will have zero or very low outputs, since actualwavelength λ_(A) is not equal or even close to any of these wavelengths,and thus nothing will be passed by reflectors/filters 1614 a, 1614 c,and 1614 d, respectively. However, λ_(A) will be still within thepassband of reflector/filter 1614 b, because it is close to λ₂. Becauseλ_(A) is not exactly equal to λ₂, less light will be passed throughfilter 1614 b than light at λ₂ would, because λ_(A) is not at the centerof the passband. In this case, circuit 1620 will be able to determinethat exact lock has not been achieved, because the signal fromphotodiode and measuring circuit 1618 b will not be maximized and/orwill not be as high as a “lock threshold” (which may be determined withreference to a power monitoring photodiode signal). In another case, theinitial actual lasing wavelength λ_(A) may be closer or equal to thewrong discrete lasing wavelength λ₁, λ₃, or λ₄, than to targetwavelength at λ₂. In this case circuit 1620 can determine this by anappropriate algorithm to vary the lasing wavelength and analyzing theresulting signals output from all four photodiode and measuring circuits1618 a, 1618 b, 1618 c, and 1618 d. Other tuning and locking algorithmsare described below with reference to FIGS. 14 and 15. As will beappreciated, these and other wavelength locking algorithms can be usedto achieve, and also to maintain, wavelength lock.

Wavelength locker circuit 1620 can also take into account expected orpredetermined losses occurring due to reflection losses at the vertices.For example, there may be more loss to the signal traveling in waveguidestructure 1612 by the time it reaches the last filter 1614 d than whenit impinged on the first filter 1614 a. Thus, circuit 1620 may take thisinto account when analyzing the signal received from each filter, i.e.it may employ a lower lock threshold for outputs further down thewaveguide structure 1612.

Thus, instead of extracting individual wavelength signals of the WDMsignal for further transmission or routing, the zigzag waveguide deviceof the present invention produces a plurality N of signals based on a(preferably single-wavelength) input signal having an unknown and/orpotentially variable wavelength λ_(A), where each of the N signals isrelated in a known manner to the intensity of light in the input signalat a particular discrete wavelength λ_(i) of a plurality N of discretewavelengths λ₁, λ₂, . . . λ_(N). In an embodiment of the presentinvention, a photosensor, such as a photodiode, is placed at the outputof each filter, to monitor the intensity of light at the wavelengthλ_(i) of the corresponding filter i. In this manner, as in thewavelength monitoring and locking techniques described above, e.g. withreference to FIG. 8, wavelength locking may be achieved using a zigzagwaveguide device.

PLC Module for Conditioning Tunable Laser Output

Planar lightwave circuits (PLCs) are used as waveguides in variousoptical applications. A PLC typically comprises a slab (substrate) ofdielectric material (e.g., silica or silicon) into which one or morewaveguides are formed or “buried”. The waveguides are usually simple andwell-defined waveguide structures, typically having a substantiallyrectangular cross section. The waveguides serve as dielectricpropagation media, for which the principle of operation is the same asthat for optical fibers that are circular in cross section. Variousparameters such as width, thickness, and refractive indices determinethe operating wavelength and the modes a PLC waveguide will support. Forexample, the PLC may be designed to have single-mode waveguides. Also,waveguides may be tapered or shaped at their ends to match the modeprofile of the fibers or other devices to which the waveguide end is tobe optically coupled.

PLCs using silica-based optical waveguides are fabricated on (embeddedor patterned in) a silicon or silica substrate by various techniques,typically a combination of flame hydrolysis deposition (FHD) andreactive ion etching (RIE). First, fine glass particles are produced inan oxy-hydrogen flame and deposited on a host substrate (Si or SiO₂).After depositing the undercladding and core glass layers, the wafer isheated to a high temperature for consolidation. The circuit (waveguide)pattern is fabricated by means of photolithography and reactive ionetching (RIE). The core ridge structures are covered with anovercladding layer and consolidated again. Various kinds of waveguidesare in use, such as N×N star couplers, N×N arrayed-waveguide grating(AWG) multiplexers and other types of AWGs, optical add/dropmultiplexers (ADMs), and N×N matrix switches. Also, the silica-basedwaveguide on a Si substrate may be used as an optical hybrid integratingplatform, since Si has highly stable mechanical and thermal propertiesthat make it suitable as an optical bench. This permits optoelectronicdevices to be formed on the PLC using precise Si “terrace” alignmentfeatures, which helps to precisely align the device with respect to thewaveguides, to minimize optical coupling losses. Further information onPLCs may be found in Katsunari Okamoto, Fundamentals of OpticalWaveguides (Academic Press, 2000), chapter 9, “Planar LightwaveCircuits,” pp. 341-400 et pass.

A PLC module having a plurality of waveguides is used in an embodimentof the invention to condition a tunable laser output. Referring now toFIG. 12, in an embodiment 1200, a PLC module 1220 is used to conditionthe output of a semiconductor laser, preferably a tunable laser, such asthe VCSEL 1210 shown in FIG. 12. The output of the VCSEL 1210 may becoupled to the PLC 1220 by a fiber or other suitable means. PLC module1220 contains a plurality of patterned silicon oxide waveguides 1221,1222, 1223 patterned into a silica or silicon slab via standard PLCfabrication techniques. In general, the waveguides 1221, 1222, 1223 maybe said to be embedded and/or patterned in a substrate. In embodimentsin which the substrate is a silicon or silica substrate, the substratemay be referred to as a silicon-based substrate.

The plurality of waveguides include a primary or main waveguide 1221 forcoupling the laser output to some conditioning device 1224 such as anSOA (semiconductor optical amplifier) and/or modulator (not shown) andthen to a fiber 1225. The waveguides also comprise a plurality ofsecondary or “splitter” waveguides 1222, 1223 for transmitting smallportions of light tapped from the main waveguide 1221. The secondarywaveguides comprise first secondary or splitter waveguide 1223 andsecond secondary or splitter waveguide 1222.

Primary waveguide 1221 has an input end input receiving light from thetunable laser 1210 and an output end for outputting said light, e.g., toa fiber 1225 for further transmission, preferably after conditioning thelight by some device in the waveguide, between its input and outputends, such as SOA 1224. Thus, the primary waveguide comprises aconditioning device between its input and output ends for conditioninglight received from the tunable laser so that the primary waveguideprovides, at its output end, light from the tunable laser conditioned bythe conditioning device.

Secondary or splitter waveguides 1222 and 1223 provide a portion oflight tapped from primary waveguide 1221 and apply it to othercomponents, such as a power monitoring photodiode 1226 and a filter1227. In an alternative embodiment, PLC 1220 comprises primary waveguide1221, secondary waveguide 1223, and filter 1227, but may not includesecondary waveguide 1222 and photodiode 1226.

FIG. 12 shows the functional relationship between the main waveguide1221 and the splitter waveguides 1222, 1223. Several structures can beused to couple portions of the VCSEL output from the main waveguide 1221to the splitter waveguides 1222, 1223. In an embodiment, the splitterwaveguides use evanescent coupling to tap off a portion of light fromthe main waveguide 1221 or another of the splitter waveguides which isitself directly or indirectly coupled to the main waveguide. In thisembodiment, the waveguides are arranged so that a portion of each of thesplitter waveguides is located in close proximity and parallel to aportion of the main waveguide. This proximity allows for evanescentwaves corresponding to the VCSEL output in the main waveguide topropagate through the splitter waveguides. Thus, in this embodiment,each secondary waveguide receives a respective portion of light, fromthe primary waveguide, by either direct or indirect evanescent couplingfrom said primary waveguide. A secondary waveguide receives itsrespective portion of light from the primary waveguide by directevanescent coupling if it is directly adjacent said primary waveguide sothat there is evanescent coupling between the two. If a secondarywaveguide receives a portion of light from another secondary waveguide(which is itself either directly or indirectly evanescently coupled tothe main waveguide), by either evanescent or other coupling, it may besaid to be indirectly evanescently coupled to the main waveguide.

In another embodiment, at least one of the splitter waveguides taps offsome light from the main waveguide 1221 by evanescent splitting and thenext taps light from this last splitter waveguide, and so forth. Forexample, second splitter waveguide 1222 taps off a portion of light frommain waveguide 1221, and first splitter waveguide 1223 taps off aportion of the light from first splitter waveguide 1222. In anotherembodiment, instead of evanescent splitting an optical tap 1215 is usedto direct portions of the VCSEL output into the splitter waveguides.

In an embodiment, the second splitter waveguide 1222 applies the tappedlight (a “first portion” of light from the main waveguide) to a powermonitoring photodiode 1226 for power monitoring purposes, and the firstsplitter waveguide 1223 applies a second portion of tapped light to awavelength filter 1227 for wavelength locking purposes. A processor 1230employs an algorithm that responds to output from the filter 1227 (andalso photodiode 1226 in an embodiment) to generate control signals whichare fed to driver circuitry 1232 by control line 1231. This may be any asuitable algorithm such as described above with reference to FIG. 16, orother types of algorithms such as ones based on or similar to aspects ofthe algorithms of FIGS. 14-15. Control line 1231 in an embodimentcarries digital signals instructing the driver circuitry how much tochange the wavelength and drive current provided to tunable VCSEL 1210.In response to these digital control signals, driver circuitry 1232generates the appropriate analog voltage and/or current signals toeffect the desired changes in tunable VCSEL 1210.

In an embodiment, filter 1227 provides at least a signal related to theintensity of light at the desired lasing wavelength λ_(L) which isimpinging on filter 1227 from first splitter waveguide 1223. Thus, forexample, if tunable laser 1210 is designed to emit at one of fourselectable wavelengths λ₁, λ₂, λ₃, λ₄, and at a given time is supposedto be emitting at wavelength λ_(L)=λ₁, filter 1227 will output a signalcorresponding to the intensity of light at wavelength λ₁. If thisreading is at a maximum and/or above a certain threshold (determinedusing the output of power monitoring photodiode 1226 as a reference, inan embodiment) then there is laser lock; if not, then processor 1230sends an appropriate control signal to VCSEL 1210 to adjust its lasingwavelength, until the desired wavelength is achieved.

As noted above with respect to the discussion of FIG. 16, in anembodiment processor 1230 is supplied with information as to which ofthe possible target lasing wavelengths (λ₁, λ₂, λ₃, λ₄) is the currenttarget lasing wavelength λ_(L). In an embodiment the filter 1227 is atunable filter, in which any of wavelengths λ₁, λ₂, λ₃, λ₄ can beselected by an appropriate control signal to be the center wavelengthfor the filter's passband. In this case, filter 1227 is tuned to filterpass light at a passband centered on the target lasing wavelength λ_(L).It may do this in response to a control signal provided to it fromprocessor 1230, or other device, to select this passband.

In an alternative embodiment, filter 1227 is not tunable but is amultiple-output filter providing a plurality N of signals relatedrespectively to the intensity of light at each of a plurality ofdiscrete wavelengths λ₁, λ₂, . . . λ_(N) which impinge on filter 1227from first splitter waveguide 1223. In this embodiment, filter 1227 hasa plurality of dedicated filters, each having a passband correspondingto one of the N discrete wavelengths at which lasing may be desired.Thus, for example, if tunable laser 1210 is designed to emit at one offour selectable wavelengths λ₁, λ₂, λ₃, λ₄, and at a given time isactually emitting at wavelength λ₁, filter 1227 will output a maximumintensity reading for wavelength λ₁ and minimal readings for the otherwavelengths λ₂, λ₃, λ₄.

In one implementation of the multiple-output filter embodiment, thefilter is formed from a reflectively coupled zigzag waveguide device asdescribed above with respect to FIG. 16.

In another implementation of the multiple-output filter embodiment, thefilter is formed from the filter shown in FIG. 13. As shown in FIG. 13,the filter portion 1227 may comprise a plurality of identical filters1314 a-d (instead of filters each having a different wavelength,corresponding to λ₁, λ₂, . . . λ_(N), as in the zigzag waveguide device1630 of FIG. 16). These filters may be distributed dielectric multilayerstack reflectors, for example, having the desiredreflectance/transmissivity characteristic. The angle at which lightimpinges on such a filter affects the wavelength passband of the filter,for example because it affects the effective layer thickness seen by thelight. In the embodiment of filter 1227 illustrated in FIG. 13, filter1227 receives the portion of light via splitter waveguide 1223 andfurther splits this light into N=4 further secondary or splitterwaveguides 1312 a-d, via tap 1310. Tap 1310 may be similar to tap 1215of FIG. 12. Each of the splitter waveguides 1312 a-d are patterned toterminate at a unique angle Φ_(i) with respect to a filter 1314 a-d, sothat the identical filter 1314 a-d combined with the angle results in aspecified wavelength passband for the filter, for wavelength lockingpurposes. That is, the angle Φ_(i) for each filter 1314 _(i) is selectedso that the filter 1314 _(i), given its multilayer stack configuration,will yield the desired passband center wavelength λ_(i). In anembodiment, angle Φ_(i) may be regarded as the angle from normal to theparallel surface of the reflector 1314 _(i).

In particular, each filter 1314 a-d has an identical dielectricmultilayer stack configuration, which provides transmission passbandscentered at λ₁, λ₂, . . . λ_(N), when light impinges on the reflector atangles of Φ_(a), Φ_(b), Φ_(c), and Φ_(d), respectively. In theembodiment illustrated, for example, Φ_(a)(0°) <Φ_(b)<Φ_(c)<Φ_(d)<90°.In an alternative embodiment, all the angles, including Φ_(a), aregreater than 0° to avoid backreflection into the source. The lighttransmitted through the specified passband of each filter is detected bya photodiode and measuring circuit 1316 a-d, and the signals transmittedto processor 1230 of FIG. 12.

Wavelength Locking Algorithms for Tunable Lasers Employing an SRBR orMRBR

Algorithms are used to lock a light source onto the peak or trough of afilter in accordance with an embodiment of the present invention. FIG.14 depicts a flowchart of a method 1400 for locking onto a peak ortrough. If λ_(i) is the desired wavelength, a particular tuningparameter of the light source is initially specified to correlate withthe wavelength band of the filter (whether it is a SRBR or MRBR) thathas a peak or trough at λ_(i). The tuning value could be, for example,temperature of the laser or the degree of pumping whether electrical oroptical. In an alternate embodiment, a separate wavelength measurementmethod can be used to tune the light source to the wavelength band. Oncethe light source is producing a wavelength within the band, the method1400 is different depending on whether λ_(i) is the peak or trough ofthe band. (A peak of a band depicted by reflectance v. wavelength is areflective maxima, while the trough of such an inverse band has areflective minima.)

For a reflective minima, the output of the filter at the initial tuningparameter specification is measured and the tuning value is then changedby a small positive step. A new output is measured at the changed tuningvalue. The new output is compared to the initial output and the newoutput becomes the initial output if it is the greater. Once onesuccessful positive step has occurred and the new output has become theinitial output at least once, the first unsuccessful positive stepthereafter indicates that the reflective minima has been over stepped byone and will result in the setting of the tuning value at the previousvalue. If no successful positive steps occur, i.e., new output is lessthan initial output for the first positive step, negative steps areattempted. The first negative step that is unsuccessful results in thesetting of the tuning value at the previous value. Thus, the algorithmtries both directions in the tuning value in an attempt to find thereflective minima and stops changing the tuning value once it has beenfound. The method for finding a reflective maxima is the same exceptthat the result of comparing the initial output and the new output isinverted. A new output that is less than the initial output isconsidered successful so that the new output replaces the initial outputand further steps in the same direction of the tuning value areattempted until a reflective maxima has been overstepped by one. In anembodiment, the method of FIG. 14 to reattain the reflective maxima orminima is periodically employed to correct any wavelength drift that hasoccurred since the last setting. In an alternative embodiment, otheralgorithms may be employed, e.g. a predictive algorithm may be employedwhen the current lasing wavelength is far away from the targetwavelength.

Algorithms are also used to move from one peak or trough minima of afilter in accordance with an embodiment of the present invention toanother such peak or trough minima. FIG. 15 depicts a flowchart of amethod for moving a laser from a present wavelength, λ_(i), to a desiredwavelength, λ_(N). The method of FIG. 15 assumes that filter wavelengthsare defined by reflectivity maxima or peaks, though wavelengths definedby reflectivity minima or troughs could be changed by inverting thecomparison steps as in FIG. 14. As in FIG. 14, the tuning parameter canbe any laser parameter that corresponds to the wavelength of lightoutput by the laser, e.g., laser temperature or pumping level. Thealgorithm determines the number of bands between λ_(i) and λ_(N). Thetuning parameter is changed by a specified value and the output ismeasured. The output is then compared to a band threshold, which is someamount of reflection that is the minimum considered to be in a band. Ifthe output is still in the current band, the tuning parameter ismodified by the specified amount repeatedly until the output indicatesthat the laser wavelength has moved out of the band. Once the output isout of a band, the tuning parameter is then modified by the specifiedvalue until the output moves into the next band as indicated bycomparing the output with the band threshold. The number of bandstraversed, which starts at one, is incremented and compared with thenumber between that was initially determined. If more bands must betraversed to reach the band containing λ_(N), the process iterates. Ifthe band containing λ_(N) has been reached, then a one directionalimplementation of the method shown in FIG. 14 is used to find thereflective maxima corresponding to λ_(N). It is preferable that thesecond specified value be smaller than the specified value used forcounting bands, because fine tuning the reflective maxima requiressmaller steps than identifying and counting the bands between thedesired wavelength and the initial wavelength. In some embodiment, thevarious bands will have different reflectivities and comparison to thoseknown reflectivities can be used instead of band counting to locate thedesired wavelength.

In the embodiments described above, the MRBR and SRBR of the presentinvention are distributed dielectric multilayer stack reflectors. Inalternative embodiments, materials other than dielectric materials maybe employed for some or all of the layers, e.g. semiconductor materials.

In the embodiments described above, the MRBR is a distributed dielectricmultilayer stack reflector. In an alternative embodiment, an alternativeMRBR may be formed by disposing two DBR mirrors on opposing surfaces ofan intervening layer, such as a piezoelectric (electrooptic) layer. Sucha reflector will form an etalon, which has a reflectivity profile havinga plurality of reflectivity peaks. Such a reflector may be referred toas a DBR-piezoelectric-etalon reflector. The reflectivity and spacing ofthe peaks depend on the reflectivity profiles of the two DBRs and theintervening piezoelectric layer, including the refractive index andthickness of the intervening piezoelectric layer. For example,comparatively low reflectivity (e.g., about 80%) dielectric DBRs may beemployed, to give rise to etalon reflectivity peaks well above 99% inreflectivity. The characteristics of such a reflector may be selected,for example, so that the reflectivity peaks fall on ITU gridwavelengths, and may be integrated into, e.g., a two-section VECSEL. Thegain spectrum may cover several of these reflectivity peaks, and may beadjusted to select lasing at one of them. Additionally, a voltage can beapplied across the piezoelectric layer to change its index of refractionand to shift the reflectivity peaks. This may be used, for example, tofine-tune the lasing wavelength when the laser is locked onto a givenone of the reflectivity peaks. Alternatively, the characteristics ofsuch a DBR-piezoelectric-etalon reflector may be selected, for example,so that they are widely spaced so that only a single reflectivity peakfalls within the gain spectrum. In this case, a voltage across thepiezoelectric layer may be varied to shift (in wavelength terms) thereflectivity peak closest to the gain spectrum maximum, to tune orchange the lasing wavelength.

In the present application, a “non-section-112(6) means” for performinga specified function is not intended to be a means under 35 U.S.C.section 112, paragraph 6, and refers to any means that performs thefunction. Such a non-section-112(6) means is in contrast to a “meansfor” element under 35 U.S.C. section 112, paragraph 6 (i.e., a“section-112(6) means”), which literally covers only the correspondingstructure, material, or acts described in the specification andequivalents thereof.

Some embodiments or aspects of the present invention can also beembodied in the form of computer-implemented processes and apparatusesfor practicing those processes. The present invention can also beembodied in the form of computer program code embodied in tangiblemedia, such as floppy diskettes, CD-ROMs, hard drives, or any othercomputer-readable storage medium, wherein, when the computer programcode is loaded into and executed by a computer, the computer becomes anapparatus for practicing the invention. The present invention can alsobe embodied in the form of computer program code, for example, whetherstored in a storage medium, loaded into and/or executed by a computer,or transmitted as a propagated computer data or other signal over sometransmission or propagation medium, such as over electrical wiring orcabling, through fiber optics, or via electromagnetic radiation, orotherwise embodied in a carrier wave, wherein, when the computer programcode is loaded into and executed by a computer, the computer becomes anapparatus for practicing the invention. When implemented on a futuregeneral-purpose microprocessor sufficient to carry out the presentinvention, the computer program code segments configure themicroprocessor to create specific logic circuits to carry out thedesired process.

The present invention, therefore, is well adapted to carry out theobjects and attain the ends and advantages mentioned, as well as othersinherent therein. While the invention has been depicted and describedand is defined by reference to particular preferred embodiments of theinvention, such references do not imply a limitation on the invention,and no such limitation is to be inferred. The invention is capable ofconsiderable modification, alteration and equivalents in form andfunction, as will occur to those ordinarily skilled in the pertinentarts. The depicted and described preferred embodiments of the inventionare exemplary only and are not exhaustive of the scope of the invention.Consequently, the invention is intended to be limited only by the spiritand scope of the appended claims (if any), giving full cognizance toequivalents in all respects.

What is claimed is:
 1. A monitored laser system, comprising: a lasercomprising a first mirror, an exit mirror, the first mirror having areflectivity less than 100% but greater than that of the exit mirror. atleast a portion of a laser cavity defined by the first mirror and theexit mirror, and an active region located in the laser cavity, theactive region containing a material that is capable of stimulatedemission at one or more wavelengths of laser light; a multiplereflectivity band reflector (MRBR) positioned either (a) behind thefirst mirror or (b) to receive an output of an optical tap positioned toreceive the laser light transmitted through the exit mirror, whereby theMRBR receives at least a portion of laser light emitted from the laserand transmits filtered laser light, the MRBR having at least first andsecond wavelength bands with reflectivity above a particularreflectivity separated by at least a third wavelength band havingreflectivity below the particular reflectivity; a first photodiodecoupled to at least a portion of the filtered laser light and producingan output based on the amount and wavelength of light received; andmeans for adjusting the emitted wavelength of the laser toward aparticular wavelength in one of the at least first, second, and thirdwavelength bands based at least in part on the output of the firstphotodiode.
 2. The monitored laser system of claim 1, wherein theparticular wavelength is in the 1500-1600 nm range.
 3. The monitoredlaser system of claim 1, further comprising a power source coupled tothe active region, wherein the means for adjusting the emittedwavelength includes a power source control.
 4. The monitored lasersystem of claim 1, wherein the means for adjusting the emittedwavelength includes a means for adjusting the temperature of the laser.5. The monitored laser system of claim 1, wherein the first mirrorincludes a plurality of layers and the means for adjusting the emittedwavelength modifies the refractive index of at least one of the layers.6. The monitored laser system of claim 1, wherein the MRBR is positionedbehind the first mirror.
 7. The monitored laser system of claim 1,wherein the MRBR is positioned to receive an output of the optical tap.8. The monitored laser system of claim 7, further comprising the opticaltap having at least a first input and first and second outputs andwherein a portion of the laser light is coupled to the first input andthe MRBR is coupled to the first output.
 9. The monitored laser systemof claim 1, further comprising a second mirror and wherein the lasercavity is a two-section laser cavity defined by the first mirror, theexit mirror, and the second mirror.
 10. The monitored laser system ofclaim 1, further comprising a second photodiode coupled to at least aportion of the laser light and producing an output based on the amountand wavelength of light received; and wherein the means for adjustingthe emitted wavelength of the laser toward a particular wavelength inone of the at least first, second, and third wavelength bands is basedat least in part on both the output of the first photodiode and theoutput of the second photodiode.
 11. The monitored laser system of claim1, wherein the MRBR has at least six wavelength bands with reflectivityabove the particular reflectivity with at least one wavelength band withreflectivity below the particular reflectivity separating eachsequential pair of the at least six wavelength bands.
 12. The monitoredlaser system of claim 11, wherein the six wavelength bands are withinthe range of 1500-1600 nm.
 13. The monitored laser system of claim 12,wherein the MRBR includes at least 60 material layers.
 14. The monitoredlaser system of claim 13, wherein the MRBR material layers include Al₂O₃layers, TiO₂ layers, and SiO₂ layers.
 15. The layer system of claim 14,wherein the MRBR material layers are based on the layer formulaEABCD(ABC)⁶⁰ABCD(ABC)⁶⁰AB with the layer identifiers corresponding tothe following: A representing Al₂O₃, B representing TiO2, C representingSiO₂, D representing Si, and E representing Al, with layers A-D havingsubstantially the same thickness after adjustment for refractive index.16. The monitored laser system of claim 1, wherein the MRBR has at leastthirteen wavelength bands with reflectivity above the particularreflectivity with at least one wavelength band with reflectivity belowthe particular reflectivity separating each sequential pair of the atleast thirteen wavelength bands.
 17. The monitored laser system of claim16, wherein the thirteen wavelength bands are within the range of1500-1600 nm.
 18. The monitored laser system of claim 17, wherein theMRBR has layers based on the layer formula ABC(ABCD)²(ABDC)⁶⁸(ABCD) withthe layer identifiers corresponding to the following: A representingAl₂O₃, B representing TiO₂, C representing SiO₂, and D representing Si,with layers A-C having substantially the same thickness after adjustmentfor refractive index and layer D having a thickness substantially 1/0.75times that of layer A after adjustment for refractive index.
 19. Themonitored laser system of claim 1, wherein the first and secondwavelength bands include wavelengths specified in the ITU grid.
 20. Themonitored laser system of claim 1, wherein the first reflectivity bandincludes a peak wavelength and the particular wavelength is equal to thepeak wavelength.
 21. The monitored laser system of claim 1, wherein thethird reflectivity band includes a trough wavelength and the particularwavelength is equal to the trough wavelength.
 22. A method formonitoring a laser system comprising the steps of: pumping an activeregion located in a laser cavity of a laser, at least a portion of thelaser cavity defined by a first mirror and an exit mirror, until laserlight is emitted, the first mirror having a reflectivity less than 100%but greater than that of the exit mirror; positioning a multiplereflectivity band reflector (MRBR) either (a) behind the first mirror or(b) to receive an output of an optical tap positioned to receive thelaser light transmitted through the exit mirror, whereby the MRBRreceives at least a portion of the emitted laser light and transmitsfiltered laser light, the MRBR having at least first and secondwavelength bands with reflectivity above a particular reflectivityseparated by at least a third wavelength band having reflectivity belowthe particular reflectivity; coupling at least a portion of the filteredlaser light to a first photodiode such that an output is produced basedon the amount and wavelength of light received by the first photodiode;and adjusting the emitted wavelength of the laser toward a desiredwavelength in one of the at least first, second, and third wavelengthbands based at least in part on the output of the first photodiode. 23.The method of claim 22, wherein the step of adjusting the emittedwavelength of the laser includes modifying the degree of pumping theactive region.
 24. The method of claim 22, wherein the step of adjustingthe emitted wavelength of the laser includes adjusting the temperatureof the laser.
 25. The method of claim 22, wherein the first mirrorincludes a plurality of layers and the step of adjusting the emittedwavelength of the laser includes modifying the refractive index of atleast one of the layers.
 26. The method of claim 22, further comprisingthe step of coupling at least a portion of the emitted laser light to asecond photodiode such that an output is produced based on the amountand wavelength of light received by the second photodiode; and whereinthe step of adjusting the emitted wavelength of the laser includesadjusting based at least in part on the output of both the first andsecond photodiodes.
 27. The method of claim 22, wherein the step ofadjusting the emitted wavelength of the laser includes measuring aninitial output of the first photodiode, modifying a tuning value by apositive step, measuring a new output of the first photodiode, andcomparing the new output to the initial output.
 28. The method of claim27, wherein the step of adjusting the emitted wavelength of the laserincludes modifying the tuning value by a negative step.
 29. The methodof claim 22, wherein the MRBR is positioned behind the first mirror. 30.The method of claim 22, wherein the MRBR is positioned to receive anoutput of the optical tap.